Some agents take at least three months to reply--it seems too early to give up yet! If you've got a new project to work on, it can help take your mind off the wait. If you've got some personal replies, it can be a good idea to pause the querying, revise according to whichever feedback feels right to you, and then go out again with queries to new agents.
As to when to quit in general (or at least put that one aside), once you've queried all the agents you think might be a good match. Or when you realize you've written a book in a genre that has gotten impossibly overstocked for the moment. Or when you've written an amazing new book that leaves the old one in the dust. The thing is, editors usually take a LOT longer than three months to respond, especially if you are unagented. (I have some mss I sent out pre-agent that took up to a YEAR to hear back on. And some I just never got a reply on.) So...just based on the time you say you've been querying and nothing else, I wouldn't throw the towel in just yet. But I'd also be working on my next book!
ETA: Stephanie, I agree with you. I think the 100 queries thing must be advice originally intended for the adult market, because I have never found 100 agents who I felt would be a good match for me (by "good match," I mean legitimate, good agents who have experience and interest in my genre, who are open to submissions, who represent the age levels of children's books I write, etc.) You want to research widely, but you also want to target well.