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.
Many editors report the deadline in their decision letter. However, I have seen some decision letters without a deadline.

Please note that most submission systems provide a Due On date. See this screenshot from Manuscript Central:



If there is no indication regarding a deadline either in the letter, or in the submission system (or the journal website), the best for you would be to contact the handling editor.