you're supposed to replace T5s in that time frame as well, and as far as I know all the bulbs work that way since they all loose their effectiveness as the time goes on.
The effective time for T5's is supposed to be about 24 months depending on the length of time they are used during the day. VHO/PC bulbs are supposed to be replaced every 6 months, this seems to be one of the reasons they are rising in popularity. This is what I have read and could be wrong, but I think its right.