As always, the answer is 'it depends'.
If you use the whole reel for yourself, and you do a one-piece, then you can likely get away with 32-35 ft per job (depending on head size, string pattern, and string type (e.g., multi stretches more than poly)).
If you do a two-piece, then you'll use at least an extra 2-3ft, depending on whether or not you use a starting knot on the crosses.
Sometimes, for a frame I am familiar with and have to do 2-piece, I'll cut for a 1-piece, but after the last pull on the long side, I'll cut just enough to tie off, then use the remaining for the crosses. Only saves me about a foot, but I cannot tell you the number of times I've thrown out the last 16-ft of a reel.