As you know, I write from a UK perspective, but actually the answer is going to be remarkably the same wherever you are, although UK agents may more often request that you don't do multiple subs, mainly because they are more often smaller or sole-traders and simply can't afford the time to read your work if they've little chance of you ultimately choosing them in a competitive situation. [Hooray for competitive situations, I hear you say.]
If, however, you think only of yourself and disobey a request for exclusivity, you'd be likely to land in deep water, piss people off and show yourself as a very dodgy person to work with. In which case, you would also end up probably not being published.
So, realise that agents and publishers are neither stupid nor cruel**. They are realistic, mostly decent and very, very busy, if they're any good. Each one is also different and you usually don't know what they're like until you approach them, so do so politely and open-mindedly, as you would anyone you wanted to do business with.
[**Actually, some are, but these are not the mainstream ones or ones that any published author has heard of. Yes, there are some oddballs, but oddballs tend to get stuck in small holes and with any luck you'll never come across them. If you think you might have found one, Google them before proceeding. It's called "due diligence" in the business world.]I started to write a list of does and don'ts, but it became too prescriptive and at the same time couldn't cover every eventuality because, to repeat, each agent or publisher is different and has a different situation. The best thing for you to do is see them all as human, in a human situation, and to realise that they DO want to find good books but keep having dross chucked at them. Accept that they understand the situation for writers, too, because all their clients are, by definition, writers. They know that we can't wait months and months for each agent to reply, and they will do their best to be reasonable. But an agent's first duty is to existing clients [and publishers to authors] and when some of the limited remaining time is spent considering a new writer's work, only to find that the new writer has secretly already approached other agents who are also all working on it, it's very galling. They're not usually salaried, remember, so they need to know whether their work is likely to pay off.
This is why, further than exhorting you to be sensible, rational, decent, and the sort of person you would like to do business with yourself, I cannot tell you more precisely what to do. I do really think you need to rely on your common sense for this. Common sense is a very important asset for a writer at any stage of his career and I can't recommend it enough.
Luckily, however, there is something to help inform your common sense. I'd like you to hop right over to this excellent post by a real agent, Mary Kole of the Andrea Brown Agency in new York. I think Mary gives a really good insight into how a "normal" agent thinks, though note that she does point out, as I have done, that everyone's different. These are not rules, just clues to common sense. It's the fact that they're all different that makes this hard to make rules for, but actually rather easy to find a way round: be decent, sensible and human and you'll get it right.
And if in doubt: be open and up-front. Ask. What is the agent going to do? Reject you because you asked a reasonable question?? I think not.
"What is the longest we should be expected to wait for a response before we could politely nudge the agent or publisher to see if they're going to reply?"I offer you my shortest answer yet: three months.
Anyway, people, I hope that helps and I hope that you're looking forward to improving your writing before you ever get to the point of submitting. Because, you know what the one big mistake writers make with their submission?
On the other hand, at some point you've got to do it... Yes, at some point you have to stop angsting and fretting, stop removing commas and putting them in again, and just send it out there. And my job over the next year is going to be to show you how to get to that point.
Stay with me!