heat mats- how long do seedlings need heat?

[davespitzer]

Maybe a silly question, but how long should I leave my seedlings on a heat mat? Just until they are up and going, or until they have true leaves, or until they're ready to transplant? I have limited space in my little greenhouse and want to start succession plantings, but there's no room on the heat mat. It's averaging 45-50°F at night, 80+ in the daytime in the greenhouse, probably mid 70's outside during the day (for now- weather's unpredictable this time of year here.) Thanks- Dave