Like others said before, depends on the group.
For my games, I try to offer 3 runs a month, with a big pay-off run every 3 or 4 months. It's up to the players if they take the runs or not. Then I factor in things like body count, happiness of the Johnson (did they talk him up too much, was he happy with the result?, etc)
If they screwed up (too many bodies, didn't get all items/data, made too much noise, etc) they could (and have!) found that there next job is coming from the people they just hit (ie: SETUP!!) or, even worse, jobs dry up for a while! Keep in mind I run campaigns, so about 50% of runs are "trash runs" 20% are story arc "red herrings" and 30% relate to the actual story arc they are currently on (love to keep them guessing as to WTF is actually happening)
In the end, pick a level of job/week ratio that works for you, and use it! It's your game!