My little man is 22 months old and we clean teeth at bathtime (night). So only once a day.

He has nearly all his teeth now (missing about 4 molars- I think!) and I let him do most of the brushing - with me finishing off if I don't think he's gotten all around.

We started after he got more than about 6 teeth... and he was eating more food that could get stuck and cause damage.

Hope that helps!