Since there are many experts who urge removing the spent fuel assemblies from the Reactor 4 Spent Fuel Pool at Fukushima I Nuclear Power Plant and into dry cask storage as soon as possible, I checked what the US regulatory agency of the nuclear power industry says about spent fuel storage to find out how soon is "as soon as possible".
The minimum the NRC has authorized is 3 years, 5 years normal, industry norm 10 years.
From NRC's webpage "Spent Fuel Storage in Pools and Dry Casks: Key Points and Questions & Answers" (part):
1. All U.S. nuclear power plants store spent nuclear fuel in “spent fuel pools.” These pools are robust constructions made of reinforced concrete several feet thick, with steel liners. The water is typically about 40 feet deep, and serves both to shield the radiation and cool the rods.
2. As the pools near capacity, utilities move some of the older spent fuel into “dry cask” storage. Fuel is typically cooled at least 5 years in the pool before transfer to cask. NRC has authorized transfer as early as 3 years; the industry norm is about 10 years.
3. The NRC believes spent fuel pools and dry casks both provide adequate protection of the public health and safety and the environment. Therefore there is no pressing safety or security reason to mandate earlier transfer of fuel from pool to cask.
The Reactor 4 Spent Fuel Pool contains fuel assemblies that were removed from the reactor core when the unit went into regular maintenance that included replacing the core shroud on November 30, 2010. The maintenance was to last until September 24, 2011 (299 days). There were 548 fuel assemblies in the reactor core at that time. (Information from TEPCO's press release, 11/29/2010)
Assuming that the fuel assemblies were removed from the core soon after the start of the maintenance and put into the SFP, say sometime in December 2010, they have been cooled in the SFP for about one and a half years by now. Another one and a half years to go, then, to reach the minimum that the NRC would authorize the removal from the SFP to dry casks.
TEPCO/Japanese government's current plan seems to be to move the older fuel assemblies in the Common Spent Fuel Pool into dry casks to make room for the Reactor 4 SFP fuel assemblies.