Hello 100pctpenn, the whole idea of determining change-out times would need to be determined by your specific application, shop conditions, environmental conditions and a host of other variables. I term it that way based on things such as wire brands and types for starters. If you are using a lot of gas-shielded flux-cored wire, you will run into various manufacturers who will use different types of manufacturing processes and this will contribute to or detract from the amounts of residual "contaminants" that could be on the surface of the wire as a direct result of it's manufacture. Depending on how much dust there might be in your various fabrication and welding areas you might also have different levels of "pick-up" of these contaminants by the dry lubricants or sticky ones that might be present on these wires. There can also be mechanical variables that can have an effect on how quickly the liners can become gummed up or worn. If the drive rolls are of an incorrect type, tensioned incorrectly, or if they are mis-aligned they can introduce shavings or other forms of contamination into the liners that could contribute to premature contact tip failure, liner clogging, or excessive wear.
     You may wish to consider determining at what point you will consider the guides that direct the wire into the feeding mechanisms should be cleaned or replaced, which types of rolls that you will be using for your various applications and making sure that you have the appropriate ones available to the welders. How you determine the correct tension for drive rolls and wire spools, many of these requirements will vary depending on wire type, size, and application. Who will be in charge of monitoring the changing, adjusting, cleaning, etc. of these various components. Generally it is best to have someone in charge of this as opposed to allowing everyone free reign to maintenance feeders/guns as they see fit. I have seen some outfits change out their liners every 1 to 2 weeks, they don't necessarily throw away the liners, instead they soak them in solvent for an extended period of time and then blow them out with high pressure air and set them aside so that they can be used again. They will also generally very carefully tag them to keep track of their age, which gun assembly they came from, and possibly even the process and wire type they were used for. The last item is listed due to possible contamination issues when a shop welds on various metals, they don't want to intermix, stainless and mild steel applications.
     You should also consider the actual liner diameters and have your folks watch to see that they are indeed using the appropriately sized liners for the particular wire diameter. Inspecting for kinks, coil separations, and other damage to the liners after they have been removed will help to prevent feeding problems down the line. Carefully inspecting both ends of the liner will generally give you an indication of the levels of internal wear that a liner has seen, if they exhibit any excessive out-of-roundness or grooving they are probably ready to be discarded and replaced. Simply taking a clean piece of appropriately sized wire and threading it through the liner section and pulling and pushing on it to see how freely it moves can be a pretty good indicator, for a comparison, use a brand new liner and perform this test first and then try it on the used one. This can possibly give you a reasonable baseline for checking your used liners.
     It is likely that your program will need to be developed from a fair amount of trial and error and documentation to prove to be successful for your particular application. I am sure that many of the others out there will have some really terrific suggestions for you, I will await to hear their suggestions as well. Good luck and best regards, Allan 

Generally, liners are changed out under 2 conditions: 1) When changing the tips and checking drive rolls doesn't cure the feed problem, or 2) when the welder decides its time for an extended tool crib visit.

Liners last a long time if they are the correct size and are taken care of.  They do accumulate "stuff" inside them (that can often be blown out) but are reliable otherwise.  The biggest reason for changeout I see is kinking from yanking the gun cable.

We have some DC-400s that were here before I statred with this company 14+ years ago.  Still going strong.  The major reason for failure is due to using them for air-arcing without connecting them properly.

We use LN-9's for the most part - it's the same as the DC-400s.  Some are many years old and still in great shape.  One new wire feeder was badly damaged when someone rolled a weldment over without checking what might be in the way first. There  is no time limit for that. 

So I have to summarize by saying that equipment abuse is the number 1 cause for failure that I have seen.  It seems that tools and equipment would last longer if you were to spray them with gray-brown paint and drag them around the yard with the fork truck. Shiny, new tools and equipment always seem to get lost, destroyed, or stolen before old stuff does.