8 bulbs total?
If it were me and I didn't have a lux meter I'd probably have two schedules since actinic bulbs don't last as long as daylight.
I'd replace a white bulb every 3 months and an actinic bulb every 2 months.
If I were running these bulbs on an IceCap ballast then I'd replace a white bulb every 4 months and an actinic bulb every 3 months since bulbs seem to be useful longer on the IceCaps.
Don't try marking the bulbs with dates. The markings often disintegrate from the combination of heat, salt and humidity (lessons learned
).
Use a spreadsheet or something to keep track of which bulb was installed and it's replacement date.
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With a lux meter I'd record the lux reading of the bulb types after they've been broken in (about 3 weeks) and then only replace the bulbs after they've lost a percentage of their lumens. 15 - 20%, or whatever you want for your "useful life threshold".