Have to agree with Tosh here. i cleaned my car windows for practice then went to do my first job!! As long as you know what a clean window looks like and that you get to know the does and don'ts with ladders, then just get stuck in. Its more thrilling (though daunting) to learn for yourself. So 2/3 days would be about right if you ask me. Some people on here talk about the skills and workmanship of the window cleaner, but na, not really, its about getting a nack more than anything. we're not talking cabinet making or engineering!
Ben