I used to do an awful lot of initial cleans for builders, one thing that I would advise is to charge at the very least THREE TIMES YOUR NORMAL PRICE!
Builders cleans are one off's.
With initial cleans for private customers who you hope will turn into regular cleans, I agree with doing the work in stages over a couple of months.
I have never used special solvents for the silicone myself, I have found that the 4inch unger razor knives are fine.
But the smaller the window panes the more frustrating it is! and of course it takes a lot longer too.
I always told customers of this sort that I would fully initial clean some of the windows this month, a few more next month, and so on until all windows were perfect.
Charge a few pound extra per clean, and then when all have been initial cleaned the price drops to your normal charge rate.
But big house or small house, initial cleaning will take you at least three times as long to do, so charge accordingly.
If the builders/glaziers/painters/brickies did their work with more care there would not be the mess to clean off in the first place!
Oh, a tip.
With regards to initial cleans, before you make a start, check the widows carefully for scratches and also point out to the builder or agent that if there is sand and cement over the widows there is a strong chance there will be scratches. If the windows are of the painted variety, check for clumsy decorators who haven't paid attention to the marks and scratches that can so easily be made from sanding down the paintwork.
The reason for pointing this out is to prevent someone trying to claim that you have caused the damage, and then trying to claim of your insurance for new glass.
Trust me, this happens!!
Ian