I have a cactus seedling (Pachycereus pringlei), it is one month old now (germinated end of August). During this time I kept the soil very moist and wet. My question is when do I switch to normal cactus soil conditions? So when to let the soil dry out between watering?
As a general rule of thumb, as soon as they get past the cotyledon (seed leaf) stage and start looking like the mature plant, start treating them like mature plants.
The one in your picture looks as if it has just reached that stage. Depending on the species, it can take anything from a few days to several months.