How often do you clean the inside of your windows?

[justtryingtodomybest]

I've just done mine and I'm just being nosey really. I try and do mine on the day the window cleaner comes so that's once a month.
At least that way there is one day when they are all clean and sparkling.
I just wondered what everyone else does as I've no idea what is standard.

Patio doors most days as I have 2 two year olds with sticky fingers. The other windows, erm, when I move house or when there is a visible mark on them.