If my Trichocereus are well calloused and rooted... I water the same day I get them. If my Trichocereus are well calloused and have no roots, I water the same day I get them.
If there is no callous and I have just cut them, I will often wait 24 hours before I plant them in soil I watered 24 hours earlier. I will expect my soil to be moist to the touch, but not damp at this point. I will allow the soil to become dry and to the touch for 24 hours and then water again. Its not an exact science and each has their own way, but I've rooted tips less than an inch in height with this method.