Personaly I have two boyz (7 and 9) who love all our snakes.
I let them handle any snakes they want.
I'd say the "how long" question goes with each individual snakes.
They can keep our ball on them for an hour watching TV, but won't handle our nervous milksnake longer than 1 minute
When my sons handled our ATBs, it's mostly to get them use (the kidz) to handle nervous snakes. They just can'.t grab them the way the grab their cornsnakes. Ours aren't nervous, but we need to be very calm when handling them and be carefull how quickly we move our hands. It's really good for them to get use to all the different kinds of snakes.
Same with the BRBs, my older son helped me once with feeding them and when putting back the male in the enclosure he got bitten.. he immediatly knew why, he forgot to clean is hands after touching a mouse. They learn really fast, we need to not put them aside to much, it,s good to have them help us, yes by keeping a good eye on them, but also by letting them learn by their mistakes also.