I've got quite a few things that all combine to keep burnout at bay.
One, my job requires travel and that FORCES some breaks - sometimes a week or two, sometimes a month, sometimes 6 months. Often by the time I get home again I'm anxious to build again.
Two, I've got lots of other hobbies. If the weather is decent, I'm as likely to be out riding the Harley, or camping, or fishing, or white whater rafting (or some combination thereof!). When my favorite sports are in season, I watch a lot of them. I'm also a pretty voracious reader. So I'm never just modelling at the exclusion of everything else for very long.... This is probably the single biggest thing that prevents burnout for me
Third, I make sure every "conversion" or detail project is followed by at least one "shake and bake" out of the box build - maybe in my normal subject field, maybe not. I just find it fun to follow up a project that took some work and extra effort with something that you can just put together. It's sort of taking a break without really taking a break.
Fourth is inspiration - whenever I pass through Seattle (I live 2 hours north of there) I stop by a hobby shop or two, see whats on sale, see what completed works are on display. Message boards and places like ARC and Hyperscale are great because you can see what everyone else is up to - they're part of my daily routine, like checking e-mail, so I'm never lacking for inspiration.
Just my $.02, but I'm very rarely totally burned out. If anything, I wish for more hours in the day to squeeze in a little more modelling!