I like 2, get them doing some easy basics, then be done till 3 or 4.
I have a 2 yr old right now, he's like a been there done that 25 yr old in his head. Majorly chill and awesome. Better than a bunch of older horses I've been on.
No serious riding still. I have saddled him, taught him to go, stop and turn. Can also get him to trot.
But really usually just get in him bareback and walk around, just to ask him to do those simple things.