Any idea roughly what I should expect to be earning after 3 months from a standing start?
This is a 'how long is a bit of string question', but considering experience is something you never have when you need it the most, I'd say you'd be lucky, and I mean lucky, if you can turnover £1K per month in the first three months, I'd say if you could, then that would be very good.
To be honest, for most of us (geniuses and whizz kids aside), it takes years to build up a good round of work with a mixture of decent sized commercial, large domestic, and compact smaller domestic properties. Controlling your cash flow is another problem (I doubt there's a window cleaner in the UK who has never been skint, yet owed a small fortune in yet-to-be-paid work).
You'll also be a bit 'flabby' having worked in an IT environment (I left the army pretty fit and strong, and I still found the work really difficult; I just wasn't used to this sort of manual labour).
I don't really have any advice for you; we struggled for a lot of years when I left a well paid job to become a window cleaner. I'm guessing that'll happen to you if you go the window cleaning route.