How long do you have from taking a plant cutting until you must plant in a medium as in plant propagation?

It really depends on the plant the cutting was taken from. Something like a succulent requires that it NOT be put in a medium immediately because the raw openning must try and seal itself in the air. Then in order to have a good root system develop before rot sets in, it merely has to be laid on top of sand, soil, or even cement. HOWEVER, a plant such as euphorbia, mums, or celibrachoa requires protection from open air and MUST stay cool until it is put in well watered soil. A good way to determine if a cutting needs to be put in the watery soil is to test the mother plant. If the leaves are thin then that usually means it has a low water content in the plant, and therefore will wrinkle and the cells will collapse.