How often do you give your dog a bath?

We're quite lazy, basically once a month... it's a struggle with our bullmastiff, who is terrified of water. We only need to say 'bathtime' and she runs into her bed, she shakes and whimpers the entire time I wash her. Our lab is much easier.
At the moment they haven't had a bath in about two months :Push: our usual shower room is being renovated (as are all our baths), I don't have the heart to use cold water...