On my Benz, I used to keep it plugged in all night, or for about 8 hours at a time. I wouldn't do this again, because the heater would last about 2 seasons at best. I plug the heater in on my Kubota about an hour before I want to start it and that has been sufficient. If it is plugged in for 2 hours, it is definitely warmer than after 1 hour, but it doesn't start any better.... I plan on getting a commercial grade time clock and using that to regulate the on / off cycle of the block heater for next winter.