With age fluorescent bulbs start to give out much less light. That's not necessarily a problem for the plants because they adapt very well to gradually chaning conditions. So you may not notice a considerable change in the plant growth as the bulbs get old.
But there will be a change and considering the prices of fluorescent bulbs you need to change them on a regular basis. 6 months seems reasonable.
If you run a bulb for a very long time and then switch to a new one you will notice how much more light the new bulb provides. The tank will look great but keep in mind that if the plants don't adapt to such a sudden change you may have issues.
Bottom line - change the bulbs on a regular basis. Don't wait to see problems with the plants or diminished light output.