I know that UVB bulbs have to be changed out every 6 to 12 months depending on the brand but what is the reasoning for this if you have adult chameleons that are fully grown. Are there other benefits to the UVB besides the whole bone development/MBD thing? Just wondering...
I actually just took my chameleon to the vet and this is why you change the bulb. Generally when a bulb burns out that means it's done producing light. With UVB bulbs just because the bulb produces light doesn't mean it is still producing UVB light. After about 6 months the light will still work but will not produce the UVB light needed. Instead try buying a mercury vapor bulb. As long as the bulb works it produces UVB light. That came straight from my vet. The mercury vapor bulbs are more expensive but the good part about them is they act as both bulbs UVA/UVB.
I'm not sure if I am reading your question right, but adult chameleons can develop MBD too. Just as human's can develop osteoporosis as their bone density diminishes with age, chameleon's bones can lose density and develop MBD at any age. UVB lights are important for their entire lives, not just while they are growing from babies to adults.
While you are deciding on Zoo Med Reptisun linear fluorescent vs. MV, here's an article that I put together that you might find helpful with your data collection and sorting things out: http://www.chameleonnews.com/10JulWeldon.html
What I was asking was, is UVB neccessary after the chams reach adulthood and are done growing. Thinking about bone density loss in humans due to a lack of calcium in their diet makes sense as to why adult chams need UVB as well to continually process calcium and maintain strong bones. I did not know adult chams could develope MBD. Thought it was a condition due to a deficency while growing. I am happy with my Reptisun 5.0 and have read only positive reviews of this particular bulb. Don't mind changing them out if it makes my chams healthy.