Depends on how many hours per day the lamps are on. GENERAL rule of thumb is based on 10-12 hours per day, replacement from 9-12 months (12 hours, 9 mos, 10 hours, 12 mos).
Something like that...
You actually got a JBJ to last long enough to wonder about bulb replacement? Wow... I went through 3 of them in as many months...