I would change them every 8-12 months or so. I also would suggest you save some money anf go with solid color bulbs i.e. 4 blues and and 4 6700k days or something, you get more intensity.
I changed out my 2 96W lights at 9 months and noticed a difference in my corals right away. It seemed to take them a few days to get used to the new light. I didn't notice them being brighter or anything but the corals did react.
Put a 6700K and a Smartlight next to each other and turn them on. The white tube of the Smartlight is significantly dimmer than a single tube of the 6700K.
Why? You got me. Maybe it's a less pure phosphor in the mixed tube or something like that, but it's a really obvious difference. And verifiable with a lux meter, but I don't have the exact numbers.
The total tube compliment is the same - If you have two lamps and use Smartlights you get:
White-Blue White-Blue
And with the regular lamps it's:
White-White Blue-Blue
But the total lumen output is not at all similar. Try it!