you could spend 1/3 of the time working snatches, 1/3 working cleans, and 1/3 working jerks.
with the time you spend on snatches you could see what you seem to need to work on. so, for instance, if you are having trouble with the first pull, then work on your first pull. 1-3 reps, as seems appropriate. a couple minutes rest between sets. or you could spend 1/2 the time working on snatches, and the other half working on cleans and jerks (or whatever part of that seems appropriate).
i used to try and work to a number of reps - but often found i ran out of time. spending 1/2 an hour on snatches worked pretty well, though. could have happily spent another 10 minutes, but that amount of time kept me focused on my rest times since i needed to make the most of the time i had.