I have read on various agent blogs to wait six months before querying the same agent with a new project. Their reasoning was to let the old project/name get completely out of their memory. You don't want to appear to have a stack of finished (which can equal unsellable to them) manuscripts you're zinging to everyone and hoping for one to stick. As for re-querying, do you mean sending the same one to someone you didn't get a reply from? Depends on the agent. If their guidelines say they respond in two weeks, and it's been three months, they either never got it, their respone never made it to you, or they never bothered to respond. If they say then can take up to six months to respond, then wait seven months then follow up with a quick and polite "what's the stauts?" note. But at the query stage, if they don't reply at all, chances are it's a no. Unless you have something to indicate they were interested just move on to the next agent on the list.
And cast a wide net when you query. I divided my list into three parts: The A-list was all the dream agents, who rep'd what I wrote, (and read) and had something that I really liked about them. They were the ones I really wanted to be with (and eventually got). The B-list were those I liked, but might not be as perfect a fit. (ie, a great agency, but they only sold a few books in my genre). The C-list was everyone else. Reputable agents I could work with who I thought would do a good job, but didn't have that special spark. Then I sent out my queries to each list, waited to hear back, then went ot the next list.
