thermostat?

My heater is blowing cold air only but it has just happened since this freezing weather, it took me 3 hrs to get it warmed up, i put the car in the sunlight and took the top of the water bottle (altho i always put enough anti freeze in) and turned the engine over every 15 mins (with water bottle cap back on) for 15 mins each time eventually she was heating up well but the problem continues and if i need the car i have to have her runing an hr b4 i take her out !