If its anything like the ones I used to clean on Aston's and Ferrari's you'll be there for a while., they hold so much crap its unreal especially if it hasn't seen a proper clean before.

I found it easier to do do a 1/4 of the roof at a time, that way you ensure the whole roof gets done well as you don't want to start sealing it and find there's so still dirt lodged in it.