The how many inches is still a little hard to answer. I think of the deposition rate in Lbs per hour as opposed to a linear dimension. Once the deposition rate of the process is calculated, then the volume of weld can be figured and a corresponding theoretical "Arc Time" can be calculated.
The 1/8" to 1/4" fillet weld is a broad range. The 1/4" being 4 times more weld than the 1/8" however the 1/4" can be welded with higher deposition rates normally since the travel speed is high for a 1/8" fillet, Semi-Auto welding would be dependent upon the welders ability to move at high travel speeds.
Another consideration is production factor. How long is the arc on in an hour? or 8 hours. This can vary widely from employee to employee, welding machine to welding machine, and product to product. The production factor is higher for a 12" weld than it is for a 6" weld and also than for a 48" weld. The welder can maximize his time with a 12" bead. Start to stop without stopping. The 6" weld requires a stop every 6". The 48" weld may require a stop every 18".
So overall, many factors besides deposition rate would have to be considered to answer the question "How long....?"
In my opinion, controlling weld sizes would be needed since with fillet welds, twice as big is four times as much.
I hope this helps.