Estimating labor costs is a crap shoot.  It will depend upon the size of the outfit, the work habits and attitude of the employees, the number of smoke breaks taken and/or tolerated, the process, the condition of the equipment, the amount of bathroom breaks for water consumption relief (lots of water intake in AZ to be released on the other side of the process), and so much more.

For larger companies with fair sized to large contracts it is easier since it becomes a matter of simple math calculations to average time of work to amount of weld completed. And there is so much info that backs up the numbers.

A shop of recent TPI work for me often checks average weld time as the time the trigger is pulled and an arc is struck which is checked by a meter easily attached to the wire feeder.  Average weld time for a welder, no fitting, on day shift was 1 1/2 - 2 hours and swing was 2-2 1/2 hours checked for a full week.  The parts consisted of 5/8 plate welded to 2 1/4" plate about 36" long, 3/8 fillet welds with studs in the way so much re-positioning, then, get the in house and TPI inspections done, roll the 40'+ long member each weighing in at 50-60,000 lbs and weld the next side.  Three sides then place, fit and weld the 4th side.  A lot of work beyond the weld time in other words.  But, that is the case with most welding, other things besides welding to be done on a shift. 

Not just a matter of calculating the weld time on the project.  There is all kinds of handling, prepping, clean up, etc.  You have to know your operation in order to even come close to calculating times and final cost.  All of which is also dependent on your rent, utilities, insurance, and all other overhead due to regional considerations for cost of each.

He Is In Control, Have a Great Day,  Brent

Depending on the complexity of the component or structure; a mock-up may be used to determine the time to lay-out,  fit-up and weld. Then multiply times 4 (minimum) and bid with that figure. The customer wants to know how much it will cost them for the weldment. They do not want to know what time you took to weld or what it cost you to make the weld. If the customer is happy with the bid you are then happy.

I really cannot answer that question without knowing exactly all of the details involved which can vary from project to project depending on not only what Brent so eloquently addressed with his response,but also other variables as well... What I can do is offer some points to consider... When one sets out to estimate a bid based on what is gathered from the take-off's alone, they're basically shooting themselves in the foot especially if they don't pay attention to any attached delivery schedules which in turn puts them in a position prior to bidding the job to ask a question or two of whether or not they have reliable supplier of raw materials, labor force and other factors that can dictate the flow and overall costs of the project... Knowing ahead of time what are all of the important variables as well as any potentially hidden ones to consider is paramount in bidding successfully on any project... Which is why it is so important to pay attention to all of the details within the scope of the project especially any variables that may indirectly affect your project requirements & responsibilities...

There's also transportation & liability costs to be considered when a project of an unusual size and shape, or any other unusual details are concerned... Because if the project turns out to require more than the usual price per weight because of the asymmetric geometry which then requires special handling, and of course the rigging and erection costs which if not carefully considered will take a bite out of any profit margin, or even swallowing it whole and one is left holding the bag so to speak... There's no such a thing as a blanket template, no cookie-cutter approach to use when estimating projects even though most - if not all of these software applications attempt to do just that...
The applications do have purpose as a time saving tool, but should not be relied upon as the final arbiter in producing the results necessary when estimating a job project...

Brent brought up some important factors regarding weld time and I just want to expand on it a bit...  For example, when one attempts to calculate the arc on time, weld time or whatever else you want to call it, one needs to be aware that an average welder is not going to spend an 8 hour shift welding for 8 hours realistically... The welder more than likely will only weld for anywhere from zero to 6-1/2 hrs or less and usually the remaining time will be the total from 1/2 hr. of lunch, 2 - 10 minute breaks and @ minimum, 2 - 5 minute bathroom breaks or more added together for shop welders... However, what if one or two are absent during the fabrication of the project or if they leave early or are injured - God forbid? And any other unforeseen circumstance which may cause a potential delay in completing the project? This is why one should bid time on an 8 hour basis @ minimum... And if there are any unusual factors involved such as having to meet a specific delivery deadline with penalties if the deadline isn't met, then I would include an additional 2 hours overtime or more to compensate, or include another welder or two to the project bid...

Remember to include more time for any other requirement that you feel may require an extra step or two in the overall assembly, fabrication, in house inspection, transportation, etc., and anything else necessary for completing the project on time... And always remember to charge the journeyman's hourly labor rate for the region even if most of your labor are only apprentices... Finally add to the bid what it costs you to insure your labor force as well as workman's compensation also...

In other words, the best formula to use is to pay attention to the requirements of the project... What exactly is your own company's responsibility to meet the requirements of the project are... And finally giving yourself a bit of a cushion to protect you from "Murphy's law" which if it rears it's ugly face, the results can be devastating to your bottom line.  So yeah, weld time, or arc on time, or whatever else you want to call it is only one factor when computing/estimating a project with all of the variables included...

Close attention must be made to include any other ancillary costs as well before bidding the project... The only way to do this is to go over the project with a fine tooth comb to look for any potential obstructions or hidden costs and if necessary, go to the site of the project and look for any potential factors that may delay the completion of the project... And don't be shy to ask a load of questions for crying out loud - even if they're not considered so relevant by the client at the time of going over the project...

All of this takes time and experience to learn and master... So, it is recommended to have someone with the necessary experience to estimate the job project especially it if entails many components, or complicated issues, or a combination of both as well as any other hidden factors that can contribute to the overall costs and affect your bottom line. You don't want to give this responsibility to someone who just wants to earn a paycheck.

Respectfully,
Henry