May vary according to your system and your BIOS version. Every time it takes 2 to 4 msecs for the refresh. Some RAMs can do it, some not. If the computer fails, turn it off.
Cycle steal actually steals a clock cycle from the CPU to do the refresh.
Cycle stretch actually delays a cycle from the processor to do the refresh. We're not really stealing a cycle, only stretching one. Hidden refresh typically doesn't stretch or steal anything. Since memory is accessed ALL of the time, ie ram, rom, etc. Causes RAM refresh to happen less often than usual.