i tend to leave my 3 week old up to half an hour. no idea if that's too long but seems acceptable to me, working off the rule that you don't leave them in a car seat for longer than 2 hours at a time. they are more stretched out in a bouncer but my DS head tends to flop forward so I don't like leaving him too long. the bouncer is used at tea time when he wakes up as I'm about to eat! keeps him happier than leaving him in his crib!