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...
I would really look into a digital phosphate test, most saltwater phosphate tests don't seem to be accurate enough to really get a good idea of how high (or low) your phosphates really are.
