I'm going to say that the drives and PLC will power up and be ready to go within 10 seconds. Now, if they are on a communications network (Ethernet/Devicenet/Controlnet) the it could take longer. Ethernet seems to be the worst.
The other factor is how long they have been off. Depending on the size of the PowerFlex, they can take a while to power down. They should also not be allowed to power up until they have completely powered down.