I weight it. Take a short roll of each type, like 2mil 15", 3mil 24", etc. where the weight would be different. Weight it and get your weight per yard. Weigh any empty cores that you have to get that value. Note that there are basically two thicknesses of cores.
Make a chart (I'd copy mine but I can't find it right now) so you only do that once and will have it from year to year when inventory comes up again.
Assuming you have a desktop scale, grab a roll, put it on the scale, get the weight, subtract the weight of the core, then divide by the weight per yard and you have the number of yards on the roll.
We only do inventory once per year, and we only weigh rolls that will have more than 4-5 yards on them. The rest is expensed.
There's a little bit of work up front in setting up your weight chart, but it's still a lot less work than rolling out everything, and it's a bit more accurate than guessing. Although I must say I've pretty much guessed the amount before the weight confirms it.