I know you’re up to here with waiting, so here’s the quick answer: three to six months. The longer version: At three months, it’s fair to send a follow-up letter asking about the status of the submission. Do that now if you haven’t already. If you’ve had no reply after six months, consider this a “no deal” and move on. It’s possible you’ll hear back after that, but not likely. Meanwhile, if you haven’t already done so, multiply submit that ms to other agents pronto, noting “multiple submission” in the query letters. No need to keep your future agent waiting on an agent who isn’t replying.