We don't really even look at time per say. We base our price off of area and difficulty. Which I guess would tie to time but that isn't how we charge.
For 10 acres I would say you can easily spray and fert it in 3.5 - 4 hours but you also need to take into count spraying along the edges and back pack blowing etc.
On a project with a lot of blacktop or concrete I will even add extra for blowing off (which again kinda comes down to time).