Alright, yet another semi-personal type of question but my 16+ years of experience and customer feedback tells the most applicable answer. Its always personal preference and we'd never push you to do anything but what YOU need and want, but here goes with the general good advice: 4 times a year is ideal. This way you get to enjoy clean windows all year round and give your house the best possible chance at staying healthy.
A minimum of 2 per year is also ok, and sometimes once a year is done. But the true benefits are often lost (though this is definitely better than not doing it at all).
it all comes down to how long you can bear to look through gloomy tainted glass and not enjoy that fresh crisp feeling and clear views. After about 6-8 weeks the build-up can begin to get noticeable, but this doesn't necessarily mean it's a waste of time (not at all actually! See our other articles on benefits of keeping your glass in good nic).
So, whatever you'd like we're happy to accomodate, but the general idea is why not give yourself a beautiful house 4 times a year?? Its not really that big an expense once you see the awesome benefits really!
Give us a try and lets see how good w can make our house look now!