I just trimmed a 15' hedge over the weekend. Aside from the height, you mentioned something that could greatly effect the number of hours. Your potential client wants them cut two feet. Make sure you look at the branch size two feet down and on each side to make sure hedge trimmers can do the job. If not, and you have to hand prune, you will be there awhile. If you are cutting two feet off of each side, as opposed to one off of each, make sure you show the client how it will look, because I assume you will be able to see through the hedge if you cut that much off
I don't mind spending money, I mind wasting money!