There's no hard answer for this. There are too many variables. It depends on how tight you tie, how many cores you're using and how long the collar is going to be.How much string do you use to make a dog collar
Best advice I can give you is to either to tie a test collar for maybe 2 inches then untie it and see how much rope it took and multiply it out or do a test collar with a known length of cord... say 6 ft and measure out how long your collar is and multiply it out. I always add some extra rope because it's easier to cut cord off then it is to lengthen it.