My soil mix is pretty simple:
any cheap and bulk cactus soil (though I have used bags of Miracle Grow) mixed at home with pumice in 30-50-70 splits depending on what I think I know about the water sensitiveness of the plant.
I agree with the ~3 year rule on refreshing the soil generally by then (though a little dependent on the original starting soil/pumice fraction) the 'soil' has become ~90% pumice with some harder to break down components left from the cactus soil portion.
Plant growth can be fast enough that a plant has to be potted up before the 3 year period is up and obviously then I refresh the soil as well.
Recently my mixes have been more 70:30 cactus soil/pumice because at greater pumice fractions the soil dries out so fast when the weather is hot (which it is a lot here) that I need to water more than once a week, which I am trying to avoid just because of the time it takes to water all my plants potted or otherwise.
