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.
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.
It all depends. Measure around the dogs neck, or measure the collar that the dog is using now, comfortably, and go off of that. Let us know the length and the knot (pattern) you want to do and we can help from there.
Measure your dogs neck, then see how many "bracelets" will equal to the diameter of his/her neck.
Like if your dogs neck is equal to say, two of the bracelets you wear, and it's not tight.You should try it
What kind of weave are you making? When I made my dogs collar I used a double cobra style weave and the cod amounted to about 12 ft for the inter weave and about 14 ft for the outer. And for size put a buckle on as if you were going to start the weave and then put it on your dog and do 2 knots while the collar is on your dog for space leave about 2 finger widths between your dogs neck and the collar and after that difficult process just finish as usual. Hope this helped!!