Not sure if anyone will see this, this late, but someone else recently posted this presentation on Rakasha, a malware based on top of coreboot open bios and friends -- so if you know the type of server, you might just pop up the cover and switch out the BIOS...:
As for time needed, assume this: The attacker sets up a physical server that can accept the same physical disks as your server (with hotswap), with an additional disk (or ssd) for booting into a hypervisor, sets parameters for this hypervisor to closely mirror that of your server. The attackers server would typically be faster than yours -- it is/can be bought some time after your server was, so this should almost be universally possible.
The attacker boots the vm server, sets everything up; then kills power to your machine, moves the disks over. Your downtime: the time it takes to move the disks (literally).
Good luck differentiating between this and legitimate downtime.
I still think reading the encryption keys from RAM after cooling them with some co2 would be the more likely attack, but it's a fun thought experiment...