I would echo A&C: If the risk of carb ice is high then leave it on all the time. You will need to lean a little more, but other than that it's one less thing to worry about.
If using intermittently, I would recommend at least 60 secs if not longer (The type I fly with carb ice risk is mainly Cessnas). If the engine roughens when carb heat is applied then you already have carb icing - see paragraph #1
Rgds
CB