I see fish and shrimp in your tank description, but I don't see corals. If you're only keeping fish, you can let the bulbs go until they burn out. If you're keeping corals, the bulbs should be replaced before the color spectrum shifts too drastically. Some people say 6 months, some say 18 months.
I have limited experience with PC's, but IMO, 9 to 12 months max on your bulbs.