How often should I change my florescent bulbs??

I've got a 35 gallon planted tank with a bunch of tetras, plecos, and a
rubber eel. Currently have a double florescent canopy with a power glow and
a flora glow bulb. I've heard that the bulbs lose a lot of thier spectrum
after awhile...and should be replaced. Is this true..and how often should
the bulbs be replaced?? Also..is there a better combo for plant growth than
power glow / flora glow??

Dave