Are they all the same - are they all in the same room - are you going to be moving gear about a lot - how heavily soiled are they - is there a water supply nearby - will the area need sheeting up.......all these factors will make that 5 hour job 7 or 8 hours.
I fell for it when my first chair job was 80 in one room, no gear to move not even a chair to move as they were all in rows.
Didn't factor the above in at the next job and my chair rate/hour plummeted but the price was the price and I couldn't put it up when I twigged after about 90 minutes.