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