How long it takes to master an album depends on how well it was mixed and how good the mastering engineer is. The more flaws done in recording & mixdown, the more work in mastering to try to fix it. A mastering engineer with an ear for the art can make a good album mix sound great. However, if you come in with something mediocre, theres only so much that can be done. Keep in mind... The mastering process is not without limitations. It CAN NOT turn crap into gold.
As far as finding the right person to master your work... I know I'm bringing in nice & solid mixes so I'm leary about allowing my work to be someone elses guinne pig. Because of that, I prefer to use people who have some history. Depending on what your needs are and budget is, its definately okay to go with someone who is lesser known and less expensive. A lot of those cats claim to be good and I'm sure its true for atleast some of them.
Most smaller places are willing to charge a set price based on total minutes and # of tracks, and will agree that the work is complete when you give final approval.