Good question Dom...
I've wondered that myself. I only breed snakes though, so I'm sure its no biggie. I have them on 14 hours on, 10 off during the feeding/breeding season (10 months). The other 2 (for brumation) I have them on 10 hours. Seems to work just fine.
Now for lizards, you got me man. Let me know if you come to any conclusions.