"in the seconds to minutes after power has been removed."
The reason is explained here:
And even the electrons from SRAM, which doesn't need refreshes and which was for decades not used as "RAM" in the computers will leak away without the power:
"SRAM exhibits data remanence but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered."
Of course, if you never power off your computer but just reset it (the power is never cut off) or if you shut it down and immediately power it up the content of the RAM can really survive for much longer.
"This study is based on 5 different computer
systems. While we demonstrate that simple warm
reset attacks (not cutting power) are effective even
against DDR3 systems, we were
able to detect
any data remanence for DDR3 after cold boots. Even
cooling the RAM chips did not reveal data remanence
beyond cold boots. This leads us to the claim that
cold boot attacks relying on RAM remanence beyond
cold boots are not possible against modern DDR3
 http://www1.cs.fau.de/filepool/projects/coldboot/fares_coldb... (search for DDR3 to find the relevant sections)
This is why it's good to power down or sdmem when you're finished working with sensitive data.
On a full shutdown persistence is not as big of a risk, as the other commenter pointed out, cold boots are mitigated by DDR3 similar to how modern SSDs with TRIM make deleted data-recovery nearly impossible (such as Swap data which may also contain encryption keys).
No. After some high-enough RAM area contained the keys and you keep it powered and your OS uses much less physical RAM than physically available there's no hard limit. Just forget the "week."