I can tell you that they are long enough to completely relax the string, without backing all the way through the barrel nuts.
Just because there are still threads engaged, doesn't mean that it is safe to shoot the bow that way!
Back the bolts out one full turn, and measure the draw weight. Repeat that and adjust to fractions of a turn as you get it close.
If your string is stretched a lot, your draw weight and draw length, are already higher than you might think. If that is the case, you need to take the bow in to a shop for a new string.