What does the owners manual say? For my Camry, the coolant should stay somewhere between L and F when the engine is cold. If the engine is cold, you could fill it till the F mark but not past it. As the engine warms up, the level will expand and you will find the coolant going well above the fill mark. If you top up the tank when the engine is cold, you will have a big mess to clean up when the engine warms up.
Also note that keeping the coolant full helps in the winter with the heater core as that tends to be at the highest point of the cooling system. Often when people complain that there is not enough heat coming out, topping up the coolant with the correct mixture should resolve that problem. If that does not help, replacing the thermostat is a good second step.