I'm finding that the colder it is, th longer time is needed. Down to 20 degrees, 3 hours usually does it, buy recently it's been down to 0 and single digits. I've been pluging in for up to 5 hours at these temps. You can tell if you've plugged in long enough based on how long the grid heaters stay on. If the block is warm enough the wait to start light will go right off after the bulb check.