I have two Reptisun 5.0 bulbs I bought at the same time...a year ago. The date purchased was April of 2003. They are still putting out usable UVB. I have another Reptisun 5.0 that is much older, but used much less before I needed it for my foster. It also puts out a nominal amount of UVB.
These are the only tubes I have had experience with the meter, but I do know of others who have needed to replace the bulbs in less than 6 months, though this is an exception and not very common. It seems the average is about 6-8 months. 6 months to be on the safe side.