What is a reasonable time to resubmit revised papers after they have been peer reviewed?

I'm pondering on how much time I should plan to finalize a paper that has received a 'revise and resubmit' letter from the journal editor.

I know that I should return it as soon as possible. However, given that I have other work to do I would like to know if there's an unwritten rule on how long should I take to resubmit.