Driveshaft length tolerances +/-?

The 49" length was an arbitrary I threw out there. Basically I'm wondering how exact the length of the driveshaft has to be as compared to the specs recommended. Let's say the DS length I need is 49", but I found one that's 49.25". These are both being measured center to center of the U-joints. What are the acceptable tolerances for driveshaft lengths?