On my Dodge Cummins I find that the glow plugs shut off like they are in a hot engine after about 2 hours. I use a timer for this. I would say it depens on how long it takes for your glow plugs to behave like they are in a hot engine. On really cold days I will double cyle them. Lat year I had to replace the cable on the block heater. I would try replacing the cable berfore the heater.

Gary T.

2000 24V Cummins111,000k
1998 E300 130,000k Still looking for the block heater, car is new to me