Applying carb heat for extended periods certainly does no harm ... except as has been pointed out, in dusty conditions ect as the air is unfiltered. I suppose it might just richen the mixture, which I guess could lead to fouled plugs ... but you'd need it on all the time to do that! Tigermoths and a few other types actually have a flap that moves as throttle is reduced to automatically apply hot air, so I'm sure they wouldn't if the warmer air was somehow dodgy!
So, (again as has been pointed out) know YOUR engine and it's particular need for carb heat.
IM