How long should it take from "we'd like to offer you representation" to "we're pitching your proposal"?
The most significant factor affecting the time it takes to pitch a proposal after an offer of representation is made by an agent is the proposal itself. I don't think I've ever signed up a client without having at least a few editorial suggestions. In extreme cases its taken six months or more to get the proposal ready, and at other times it's only taken a few weeks. Sometimes the delay is a result of something out of our control - perhaps you need an interview from someone for the sample chapter but they're out of town for a few months. Other times it's simply because the author has yet to really nail it.
It's a different story if the agent is the cause of the delay - whether by not getting back to you quickly enough with edits or simply not submitting. We're busy people, but truthfully our clients come before anything else we have going on, so an extended delay is really not acceptable. Before reacting too hastily though, make sure to bring up the issue with your agent and get their response. In any situation there can be unforeseen and unusual problems that can cause a delay that might be excusable. And even if there's nothing like that going on, a bit of leeway is fair. But if your agent (or their assistant) can't get back to you within a week or so - even if it's to say I'm still working on getting you an answer - than I think you should be concerned.
Another possibility is that the agent proscribes to the dead summer theory - i.e. not submitting in July and August because lots of editors are on vacation. I personally don't follow this rule and have sold plenty of books in those months, but I know quite a few good agents who do.