Maximum I would think 2 days work
You are going to have to be really careful as 4200psi is alot of pressure for the siding.Alot!
I wouldn't guarantee that you can get the stains off.Black stains on gutters will not come off with pressure cleaning you have to physically scrub them off.So keep that in mind if you were only planning to pressure wash.Might want to put that in writing.
If it's mildew it will come off with a light solution of chlorine.
Remember that chlorine can and probably will kill the plants and sections of the lawn.Rinse with water before you apply chlorine and rinse well after.Might be a good idea to have a helper for this.Make sure all the windows are closed completely, and what about peoples patios? and all their little ornaments? Are you going to clean the patios or the ceilings above? (that will take longer)Schedule the days of cleaning so people can remove all their furniture and stuff.Let them know what side of the building you will begin working on and the time.Posting it is a good idea.
It's simple-what's your time worth? what do you think you want to make by the hour (don't forget to add money for the helper)
My pressure cleaner needs filled up every 2.5 hours.Chlorine is $3-4 per gallon and you need an adapter to be able to mist in on.(do you have one?)