RecepBilgin on the Permissions page, you find the Allow post editing option.
The offered options are Indefinitely, For 10 minutes and Until next reply.
To set a custom duration, you can find the allow_post_editing row in the settings table and set another number in minutes. By default that column will contain -1 (for indefinitely), 10 (for 10 minutes) or reply.
I have not actually tested it but I believe any (integer) number of minutes can be entered manually there.