thermodynamics

hi... i want to solve a problem...A room is 3x5x6 meters and initially at T = 0C. How long will it take a 1 kW electric heater to raise the air temperature to 20C? Note: In reality it will take several times this long because the walls and furnishings in the room have to be warmed up also.ignore it..i need the calculation for ideal condition. welcome all commends ... thanks in advance...