A golden rule is obviously to have a contract in front of you. Most of the time you can expect to get this quickly (3 days?), but internal and external post systems vary, even if the sender is quick. It might take a bit longer for perfectly understandable reasons (internal procedures require someone high up to sign it off, they agreed to it but are out of the country, and numerous other possibilities). After about 7-10 days it is starting to look a concern, but you should put a few polite calls/emails through to check. You could then keep doing this at reasonable intervals until they respond. After 4-6 weeks there would have to be something fairly exceptional for it to be very likely, but higher up the pecking order things can sometimes be much slower, and indeed quicker.