Rock polishing is an activity that takes some artistic ability, some engineering discipline and a great amount of patience.
It takes a little artistry to imagine what a rock will look like after you have ground off half its volume, then smoothed and shined its surface to where it is smoother than a marble. The engineering disciple comes in taking care of the machines (bearings and motors), keeping everything spic-an-span clean, and keeping careful notes on each step of the process. Finally, patience is required as you are shepherding a process that you can't see for 99.999% of the process - and you can't rush it. Many of my runs have spanned a million turns of the drum and that makes several million turns for each stone.
I buy some of my rocks and I find some of my rocks.
If you want to buy rocks, I believe there is no place as good as The Rock Shed in Keystone, South Dakota. I have purchased rocks from other places and none of the other suppliers came close to the size, color and consistency of rough from the Rock Shed. Their shipping is quick and they always save you money.
I have tumbled rocks from my back yard and rocks picked up while visiting friends and family. There are some pretty amazing rocks out there but the look in a person's eyes as you hand them a silky smooth rock from their own back yard has no match.
Wherever you get your rocks, be sure that your start with enough rocks to equal twice the volume of the finished rocks you want. You will grind away about half of the original rocks and the tumblers work best when loaded 5/8 to 3/4 full - so start two batches of rough and combine or have some extra stones to mix in to add back volume. I use plastic beads whenever using 500 grit or finer compounds. This has helped me reduce frosting on edges.
The best place, hands down,for buying tumblers is the Hobby Warehouse. Their prices are always the best and they seem to be able to keep a good stock on hand. If you find a better price someplace else, buy it and you will have gotten a real bargain.
One other recommendation on shopping for tumblers - don't be distracted by a package deal: a tumbler, some rock and a polishing kit. Almost uniformly, the there is not enough rock for a tumbler load and there is never enough grit and polish to produce smooth shiny rocks. Buy a good machine and get some rock and polishing supplies from The Rock Shed.
For tumbling grit, polish and spare parts, I usually shop at The Rock Shed.
The only plastic beads that I use are the Thumler beads - they are smooth, spherical plastic beads that float nicely in water. There are other beads out there that look like drops of plastic. These beads float fine but because they have a concave face, they stick to anything and everything. They are just not worth the pain. Order your beads, Thumbler tumbing beads, from the Hobby Warehouse.
What works for you will be a process developed by you. I read many sites on the internet and tried many "recipes". Some folks use plastic beads, some don't; some mix a sugary syrup, others have had disaster with that. What is consistent always is you must keep everything very, very, very clean so that you don't introduce unwanted sources of abrasion. These abrasive sources can be grit from a previous cycle or shards from rocks that shatter in the current cycle. I have learned to set aside any rock that fractures or looks like it might fracture or that has a hole that fills with grit or that has a pocket or a crack bigger than a hairline.
The smart alec answer is 'it depends' but that is true.
It depends on what kind of rocks, how big the pieces are, how the size of rocks varies in the drum, the size and kind of grit you are using, how frequently you change the grit, and just how fine a finish you want.
There are many places you can read that map a four step process that takes about a week per step. I have never been able to get good results on that path. I usually grind for 4 to 6 weeks in the rough grinding grit, as I like smoothly rounded stones. I change grit every 7 to 10 days.
A rock tumbler is a machine and like any machine it will work better and run longer with good care.
Always keep it clean - really clean. Wipe up any drips, spills or leaks as soon as you see them - a paper towel works fine. I wax the deck and uprights on my machines with automotive paste wax. I do this as they come out of the box and then I wash and rewax them once a month. This really makes wiping up a lot easier.
I clean the shafts and bearing each and every time I pull a drum to change grit - so, every week to 10 days. Wipe the parts with a paper towel - there is usually a fair amount of black "wear grease" that you don't want to get on your clothes - and then lubricate with something like 3 in 1 oil. One drop of oil on the motor bearing. I clean the belt and pulleys with a towel that is wet with Windex and make sure to avoid getting any of the fresh oil on the drive assembly.
The lids wear out. The first indication of this is a softening in the center. I replace the lid on my A-R12 (12 pounder) every 6 to 8 weeks. I missed this once, and you get this mess. The lids on my A-R2 (3 pound drums) run longer. The rubber seal inside the lid on my Model B (15 pounder) wears out about once a month. I got tired of replacing the rubber so now I make plexiglass lid liners that go inside the rubber seal (next to the rocks) and replace them every 6 to 8 weeks.