I take the entire dose in one hit. By the time I have put the pipe down, I am losing contact with my body fast. It's the only way for me, taking multiple hits just gets confusing and becomes a hassle. Also, it is worth noting that taking 3 hits from a 20mg dose will produce a markedly different (less intense) trip than 20mg taken in one hit. Other factors come into play, such as the speed of inhalation and how long (each) hit was held in. One toke, held for 20 seconds. BOOM.
Sometimes I will have a Changa meditation session, where the goal isn't a journey to hyperspace, but a lower level, deep meditation. In these circumstances, I will take a 10-15mg puff every 10-20mins to keep the ball rolling. This I find is very centering, and lovely experience in its self. Depends what you are looking for IMO...