bbPress disables topics and replies editing for normal users after 5 minutes by default.
bbPress before 2.6
Only keymasters and moderators can edit topics or replies after the editing is disabled for regular users. You can change the edit expiration time in the WordPress Settings -> bbPress -> Disallow editing after. You can only change the time limit, but this feature can’t be disabled.
bbPress 2.6 or newer
This has been changed, and in bbPress 2.6 you can set this value to 0, allowing editing to always work. This option is under Settings -> Forums -> Forum User Settings -> Editing.