There's a
blue postthat explicitly says that the 12h rotation is for beta testing, and the real rotation is weekly.
Basically, every week, you are going to have a combo of specific affixes; everyone is going to have the same combo for that week, and the combos rotate on a fixed basis. You start with a tier 1 stone, you get that week's first affix on your 3rd successful run, you get the week's second affix on the 6th run. So it's not random combos, there's a list of possible combos, and you'll know in advance what you'll face this week (and have easy weeks and hard weeks. Up to you to see if you try to get that week's big loot or not bother after 5th run).