RE: Cleaning with tumbler
How long the media lasts depends a lot on how dirty the brass is. I use a 50/50 mixture of fine ground corn cobs and fine ground walnut hulls, both of which I get at Petco because it's FAR cheaper than the media next to the reloading supplies at your local gun store. I use a 50/50 mix because the walnut hulls scrub off the worst fouling and tarnish, while the softer corn cobs polish the brass to a high shine. I also add about 1 oz of Iosso Brass Polish to new media and run the tumbler to mix it for about 1/2 hour before I put any brass in, and then I use a capful each time I polish brass to refresh the media. The media is good until the corn cob media turns grey/black, which takes a long time since I try to keep my brass clean (i.e. dry and off the ground).
As far as sizing/depriming before or after tumbling, that's really a matter of preference, but it is a very good idea to tumble brass before it goes through your sizing die to prevent marring and wear on the brass and die from trapped abrasive debris, as well as to minimise the chances of a stuck case. I prefer to deprime before tumbling as well, and if you do you'd be wise to invest in a universal decapping die (they are cheap). Just make sure that you check each primer pocket and flash hole for media and clear it out before priming as media will find a way to jam itself into those places.
Mike