We use several of them, mainly for Inconel alloys. As Shaun says, they will not remove anything more than light burrs, so parts with heavy dross must be hit with a belt sander before being tossed in. Cycle times will vary greatly based on piece size, material and media, but our cycles for Inconel run from 2-5 hours normally. Aluminum is closer to a half hour.
Note that this isn't the greatest process if you're going to paint or weld right after...the ground stone slurry created in the tumbler gets into all the crevices and laser grooves in the part. Welding is dirty unless all the edges are cleaned thoroughly afterward.