Hi Daniel
Some good advice above. I think it depends how hungry you are for it. Forget facebook and other forms of online marketing.
Go door knocking every day from 16:00 until 19:00 and leaflet outside of those hours. Speak to other window cleaners in your area and see if they know of anyone who either has too much work or is about to retire etc.
If you do the above, you are practically guaranteed to have new work every day. Set yourself a target of so many leaflets per day and so many first cleans/customers a day. Take on the one off jobs that nobody else wants, speak to the shop owners, property management companies etc. I personally would avoid estate agents due to bad experiences almost every time.
If you do three hours knocking every day you will get absolutely minimum, four new customers a night. Clean them the next day and repeat.