Just go by what momma cats lets you. If she seems comfortable with it then go ahead. If she seems anxious and overly watchfull or if she meows at you or the kitten, hand them back to momma. I've held Kiera's kittens and Chanel's kittens (fosters) since they were born.
One thing I would recommend though is one at a time. When Chanel's kittens were about 3 weeks old, we had a bunch of kids come over to see that kittens and the kittens got passed around the room. Momma was not too impressed (and I should have known better). The next day she moved them under the stairs ~ Learnt the hard way on that one !
Enjoy your new kittens