I two main factors (and some minor ones) determine how much time passes between runs. The minor factors are things like injuries, healing time, training time, etc., but I have been known to start a run before these are finished, and just add time after the run to the process to complete.
The main factors are
1) I started my game in the 2050's, with 1st edition mods, and I am working to get through some of the major Lore adventures. My group is new to shadowrun, and knows nothing (except minor blurbs in the history section) about some of the major happenings in the world. So, I am picking and choosing some of the ones that drop the biggest bombs in the metaplot. In any case, the main factor of this is that I skip large periods of time, as I still need to reach the 2060's, etc.
2) Success/Failure of the previous run. After Mercurial, their names were pretty heavy on the streets, so a lot of the Fixers with subtle jobs passed them by. Recently, I had a mini side job to fill a short session. It was a friendly bet between two opposing high ups in the military. One was paying off a bet with a tanker truck of beer, the receiving end was waiting to ambush the Runners with a brawl before the beer could be delivered, and then they could all partake in some of the beer together, but one of the Runners had to bring out an assault rifle and threaten the opposition.. So, their Employer (and the Fixer contact that arranged the job)) were very unhappy with them after that, so more jobs and time passed them by.
tl:dr How they performed on their previous job(s) is one of the main factors in how much time I have pass before someone seeks them out for a job