You can pull how ever often you want. In my case, it just depends on how much time and when I can find without the family asking me a kazillion questions. I always get one done in the morning when I shower. If I take lunch alone, I'll do one before lunch time (may actually bring my lunch back to work to eat on the job). I'll try to get a third in the evening, more back to back then if the kids are in bed and the hubby is out "playing" in his workshop. I have to put in all-nighters on Tuesdays for the job (work I bring home), so I will do numerous back to back while working on the computer, basically stopping when I get bored with it (this can be up to 4 or 5 times in a row).
Usually I go 20 minutes, but I base it on when the oil goes from thick to thin. Anymore, it takes longer for the oil to get thick. I think it helps if you are well hydrated before you start oil pulling ... it'll get thicker quicker. I'm guessing it's that your body is more likely to let go of the toxins if you're not dehydrated.