From what I have been able to find online, when I was researching T5HO bulbs a few years back, is as soon as you turn the bulb on it will start to lose strength. The pros replace their bulbs around every 4 months, but that does not mean you have too!! Personally I have some bulbs that are nearing 3 years of age, yeah that's a bit excessive but up until recently they have worked amazingly. Now I can tell it is time to replace them because the plants in the back of my aquarium (I've two fixtures one in front and one in back six bulbs in all) do not have as deep of color as they should and I am able to notice.
Since this question is more about matter of opinion, here's my opinion; replace the bulbs when they burn out, or when you feel like it.