In my (very limited) experience, some commissions are great fun, and some are agony. I will only accept commissions that tickle my Muse and really interest me, because I know how much I drag my feet on projects that I'm not enjoying, and that's not fair to the customer. Unless, of course, the customer doesn't mind waiting, which brings us to your questions!
I *always* agree on the price up front, and require a nonrefundable deposit up front as well (usually half the total, minus shipping). That way, I won't put lots of time and energy into a project only to have the customer change their mind. No loss for them if they haven't paid anything, but a big loss for me unless the piece is something I can easily sell to someone else. (I once made the mistake of not requiring a deposit from a friend, and I never saw the money. Lost the friend, too. Needless to say, I am very wary of working for friends now.)
I think it is a good idea to agree on the time frame up front as well. This is mostly for the customer's sake, so the artist doesn't take forever to finish, but it also benefits the artist… because the artist doesn't take forever to finish.

I know that if I don't have a deadline, AND I'm not enjoying a project, I will find anything else to do instead…. and I hate that feeling of a project hanging over my head.
If you find you simply can't finish by the deadline, make sure you communicate with your customer. Discuss a new timeframe and do everything in your power to honor it. I will bust my butt to honor a deadline -- which is why I hate deadlines -- but I know they are necessary, at least for me.