I did an experiment to hold an IST-ADA departure on March 5th. Flexible fare V.

The confirmation email said the reservation would be held till Feb 9th. However I do not see the detailed information as jerryhung posted.

Since the Flexible only give free cancellation up to 12 hours before departure and that is useless to us as we are flying in JNB-DOH-IST so we would still be en-route within that 12 hours window.

I notice the Promotional fare also have the Pay and Fly option too. Went ahead to book the promotional - the confirmation email said this

You must purchase your ticket until 05:00 24.DecemberİSTANBUL local time. The reservation will be automatically cancelled if you do not purchase your ticket within the time limit.

So it is a 22 hrs hold as current time at IST is 06:20 12/23 as the time stamp of the reservation is Transaction Date: 23.12.2014 06:16.
Strange why it is 22 hrs not 24 hrs.

I guess one can roll the reservation as long as the fare still available, but one must cancel the earlier booking first before rebook. Else the system either would not allow it or cancel the duplicate automatically. So a manual cancellation initiate by oneself is needed.

PS:
Made another reservation Bodrum to IST, again promotional fare. Got the same deadline to ticket at 05:00 24 December Bodrum time, with the time stamp of the reservation being Transaction Date: 23.12.2014 06:39

The flight options screen on this route looks very different from the IST-ADA route, or the IST-ATY route - it is as i am redirected to another site.

There is also a glitch with the site, as when I went from 1st reservation back to Home page to book a second flight, despite the Home page functioned normally, it pulled up a blank screen after I submitted the flight details. Started from a new window yielded the same blank page. I had to delete all cookies from TK and THY website then the reservation functioned again.