I've had a spate of queries from writers who are looking for a new agent. Some I'm eager to sign; some I know are a bad match instantly.
I think I'm able to get a sense of whether a client will be a good match pretty quickly --after all, I've got 47 clients, and there are a few formers running around too. In other words, I've had 50+ chances to see what worked and what didn't.
For writers, you don't want to come close to that many decisions! We hope you'll sign well the first time. If not, hopefully the second.
Here's a good post at BookEnds about how one of Jessica's clients decided how and with whom to sign the second time around.