For future reference:
He can estimate all he wants. Get a firm, agreed upon price and sign a contract. That should be the price you pay whether it takes his crew 3 days or 3 weeks to do the job. If you're paying by the day, you better believe they're going to stretch it out an extra day if possible.
Now, to cover his ass, expect that there will be a "time and materials" clause in the contract in the event that they run into something unforeseen, and make sure that the contract is written such that you must be contact before he begins any work under this clause. Tree removal should be pretty straight forward though.
Side bar:
I sat on the board of directors for my local gun club for a number of years. We were doing some renovations to the ranges and knew we had very large rocks that would need to be moved. The job price was quoted at a fixed price PLUS time and materials (at an agreed upon rate) to break up any rocks that could not be moved by their equipment as they were.