Thanks everyone, we have been trying DD2 in the Bumbo, and she will last a little while - 5-8 minuntes at a time, she gets a bit slumpy, so don't want to do it too long until she improves her trunk control a bit more, but she does love it until she gets grumpy :)
I think jack was about three to four months. That was more to do with win we got it tho. It lasted til he worked put how to escape out of it at six months.
A friend of mine tarted using hers at 6 weeks! Strong bubba. She would leave her in for as long as she looked comfortable.