Training happens 24 / 7 / 365. Whether it is a conscious effort by you, or not. Just something to keep in mind.
Of course, I will do short sessions in the typical sense. Usually for basic behaviours such as sit, stay, recall etc, and usually while I'm stationed at the kitchen table or on the couch.. those kinds of sessions. But I'll also capitalize on the many, many other opportunities and "trainable moments" that occur steadily throughout each day. IMO keeping a pocketful of treats on me at all times during puppyhood, *just in case*, will really assist the learning process. The intent is to instill a belief right from the very beginning, that treats can possibly be earned anytime and anywhere (not only at the kitchen table or couch). This way I can easily capture all of the spontaneous moments, and also generalization of the basic behaviours can eventually take place as well.
As far as number and length of 'structured' sessions. I'd say between 6 to 8 per day, for an absolute maximum length of ~ 3 minutes each time. Plus a similarly reasonable amount of spontaneous stuff on top of that. Aim to keep training fun, fresh and lighthearted, and never over-drill.