Thanks everyone, TK gave me 72 hours on buy now/pay later option, but I can't work out if this guarantees the fare.
The confirmation email I received states that TK reserve the right to change the fare during the window between "booking" and "issuing ticket" which I guess this 72 hour window falls under.
When I called TK reservations, the agent seemed sure that the fare will be fixed for the 72 hours window (also confirmed no 24 hour cancellation window).
Doesn't look like there is an official published way to offer the minimum 24 hour hold for fares/seats on flights departing anywhere including USA.

Let me ask another way, anyone ever faced a fare change during the "hold" period using the buy now/pay later option?

PS> Hold booking is fine for my visa application, in fact they insist not to pay for flights until visa is issued!