I'd say about 9-12 months for the halides and about 18 months for T5s. If you have a fixture with multiple bulbs do not replace all bulbs at the same time but replace a one/few at a time over a period of weeks as not to shock inhabitants. I currently have a fixture with 12 t5s and when the time comes (end of the year) I will change 2-4 bulbs at a time, with newer bulbs being installed every two weeks until all 12 get replaced.
Actinic bulbs however usually don't last as long as 10K bulbs and some people recommend replacing as soon as after 12-14 months. Still much better than changing T8 or T12 every 9 months.
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