I don't plug ours in until we intend to use it. If it's really cold, <20 degrees, I plug it in the night before, otherwise about 3 hours is enough to get everything flowing for an easy start.
Don't forget to turn it off after starting, ours is on an inverter circuit and will drain the battery quickly without shore power or the engine running.
Denny, Connie & Shadow (former barn cat made good)
2006 Tiffin Phaeton, Cat C7, Freightliner Chassis