As said before, a day's rate and dont take it personally if you're knocked back, there's plenty of people out there who appreciate a valuable service like window cleaning and others who expect a days work done for fifty quid. If you get it happy days, at least you'll not be cursing the job for having priced it too low.
I'm fortunate enough to have a fellow window cleaner's number who's happy enough to take on builders cleans and gutter cleaning etc and I pass work on to him regularly. Ideally you'd be best off trying to find similar as these cleans are a complete pain in the backside.