(UPDATE) According to TEPCO, it took 30 minutes from the moment the bottom of the cask touched the water in the cask pit till the cask bottom is securely at the bottom of the cask pit in the SFP.
TEPCO released the high-resolution (for TEPCO, at least) photographs of the cask being submerged in the cask pit of the Spent Fuel Pool in Reactor 4 at Fukushima I Nuclear Power Plant.
According to the schedule, they should soon start removing the first fuel assembly from the pool into the cask.
From TEPCO, 11/18/2013:
(Click to enlarge. For high-resolution photos, follow the TEPCO link.)