The last time I asked that question most replies were close to or at 10 months.
Something about the coating breaking down on the inside of the bulb and changing
the quality of the light given to the worst/less correct spectrum.
I find that those answers which said it don't matter..they had multiple tanks to buy
bulbs for. You can convince yourself of anything that you think you need to.
Problem is...which side is doing that ?
If they are just standard 10000k bulbs, pick any manufacturer other than Coralife. You can replace bulbs as much or as little as you like. It is a personal pref. If you see a decline in how your plants grow change them. I know some that have gone over 2yrs and still going.
10-12 months is usually the norm from manufacturers. Take that with a grain of salt, though as YMMV. There will be a shift in the spectrum (as noted above), but not too crazy.
I usually give them a year before changing them, personally. The lamps that get replaced go into a box in the garage in case I ever need one in an emergency. IME, after about a year, I start to get algae to stick around a little bit more than usual so that's when I change them.
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.