You can proceed before the fixed time is expired by pressing the Escape key. That function is intended for experimenters who want to review the session while they are developing it and don't want to sit there for five minutes waiting for the duration to expire!

So, normally, subjects would be unaware of the Escape key option, but you could tell them about it if you actually wanted to provide that as an option. I think the first (self-esteem questionnaire) sample that comes with MediaLab demonstrates this on its essay item.

Another option would be to create a custom HTML item--it would be more work, but then you could do it in pretty much any way you like.