I don't think there is an exact answer.
Mine, on my Kestrel is about 60". The way I made it allows my bridge to be adjusted from 32" all the way down to around 10 inches...I'll never need it to be adjusted to either of those extremes, but I just had the ~60" piece laying around so that's what I used. I just made one for my buddy's Mantis but we really didn't measure. We just laid out the saddle and doubled the length of the Amsteel, then added a little.
The location of the whoopie bury will dictate a lot of how much you need.
I can explain better a little later, but right now, as I type, I'm coming down with a migraine headache and I'm losing vision. These headaches suck.