Hi Leewood
There are a lot of factors to consider.
The first is probably location. You'll gradually get to know what the going rate is in your area, either per hour, or per 3 bed semi etc. Customers will tell you what they were paying, other w/cs in your area will tel you...up to a point, and you will soon work out what you are worth, by the speed and standard you can work to.
Don't be greedy, be honest, hardworking and reliable.
Let some people on here know what area you are in. If you really need to know something, there's probably someone not too far away who will give you good advice.
Finally, don't believe all the hype. Windowcleaning never has been, and never wil be an easy job, if you are the man on the end of the tools. It's a manual job, and you can only work as fast as your hands will let you.
Pj