You've got a few options, Rayhzel.
When it comes to duration, I've done three different approaches. 1, the dice roll method that the book outlines. 2, a specific amount of time for swings/snatches that is the same after each session (usually 10 or 12 minutes for me). 3, swings according to the S and S program, so 10 sets of 10 one handed swings, rest as much as needed, and go for power.
Sometimes I have been on the program where I use the same weight for everything, but usually I find myself using different weights for the swings/snatches. My last go around I did presses with the 28, swings with the 32, and snatches with the 24. On other occasions I've made the med. and heavy swing days different by the weight of the bell rather than number or reps or time.
Do the swings and snatches for sure, but remember that the program is really about the clean and presses. I think somewhere in the book Pavel mentions that the swings are like a carpet bombing on your body and conditioning systems, they fill in all the gaps that the clean and presses might miss.
How have you been doing your swings?