S.T.
My 7 year old lost a lot of teeth at once (he might have been 6 at the time), and one of his front teeth took a long time to grow in (and in fact it is still growing it), I would say at least 5 months (I was just going to talk to a dentist about it and then it started to grow in!).
One thing I noticed is that this tooth started to come in at the same time the one next to it fell out. So, it seems that teeth grow in spurts.
I am not sure what is the normal time frame,,,,I don't think it took this long for my other child's teeth to grow in!