I leave for work at about 7:00 am and i have a timer hooked up so the block heater goes on at about 3:00 am. The valve cover is warm to the touch when I leave.

These block heaters really work well. I once took temp of the block in the morning after the block heater had been on all night.. overnight temps were in the 10-15 deg. F range, and at 7:00 am the temp of the block at the front of the engine just below the head was about 68 degrees!!! Toasty! (I measured temp with a magnetic surface thermometer used for hvac ducts)