It varies a lot; in California the legal maximum is two months' rent, and it's fairly common for landlords to demand the maximum. That can be a significant hit unless you stay in the same place for years.

In Britain these days, the deposit is held by an independent agency, not the landlord or letting agent. They have to give a good reason to have money deducted - and you can then dispute this if it is spurious.

I recently moved, and the landlord (who was broke) was trying to keep money for all sorts of non-existent faults, most of which were due to his shoddy DIY work before I moved in. I raised a dispute and eventually got the vast majority of the money back. Just make sure you keep pictures of the state of the house before you move your stuff in.