Driveshaft length tolerances +/-?

Hello all, I have a question about driveshaft length and the tolerances relating to such. The question is pretty simple - how much tolerance is acceptable in regards to proper length of the driveshaft? In other words, say the specs for a particular year/body/trans/rear call for a 49" driveshaft, what are the acceptable tolerances for said dimension? Is 49.50 unacceptable? 49.30? 49.10? Etc., etc.

Obviously, we would want the length to be as close to perfect as possible, but HOW close is close? Are we talking thousandths of an inch? Fractions of an inch? And why? I understand the yoke needs some room to move in and out, but how much room does it need? Is too short better than too long?