It is different for anyone. General BDSM rules apply - make sure you are aware of the state of mind and body of the sub at all times. Check for any signs of poor circulation - change in skin colour, tingles, too much warmth or cold on an area and so on. Also make sure the bonds are not too tight and are properly supported (i.e. a proper support harness and ring set rather than tying rope to a random ceiling fitting, using proper cuffs that spread the weight rather than rope which cuts into the skin a lot). You may want more than one point of support so that not all the weight is on the wrists - so a body harness. Also make sure that the feet can touch the ground for at least some of the time. It is ok to take the weight off the feet for a short time but not too long