You should air out your house at least once per week when it's possible to do so.
In the winter it can be hard to air out your house because you need to keep the heat in the house and the cold air outside.
However if possible turn the heat down for 20 minutes or so and open the windows or at least a window for 20 minutes and allow some fresh air to come in.
That helps to get rid of stale air in the house and bring fresh air inside which is healthy for you especially during flu season.
Being cooped up in the house during the winter means viruses can easily spread around and make you sick so by having some fresh air inside the house you keep that stale air outside.
When it's warm out but not too hot I always keep my windows and doors open as much as possible when home.
But when it gets too hot then I need my air conditioner and so I shut the windows and doors.
When spring comes though like now it's warm out I have my windows open to allow fresh air inside the house.