One run per week in-game time sounds rather much. You pay 2 grand in lifestyle, maybe 1k in expenses on a run, and the default run with somewhat-decent opponents already pays 12k, 13k or more after Negotiation, and 6 karma. That's 10k profit a month. If you're doing four runs per month, you're doing about 45k and 24 karma a month. Meanwhile, we got big heavy NPCs in a book living on 6 karma a month over a total timespan of 4 decades.
And interesting way to look at it. On one hand I'd say you definitely shouldn't rush the PCs through growth (because that takes a bit of the satisfaction away) but on the other, they're still doing the work so it's not exactly a free lunch.
I could see one run week if your runners were making like 5-6k a run, doing stuff like "chump" work mainly for gangers and stuff. I mean... There's nothing that really says anywhere that one run a week is impossible, just that I suppose it may be a bit of a stretch to assume your fixer is able to find so many jobs for you. Again though, if you're just working for various gangs or syndicates doing relatively small-time stuff (because looking at the suggested pay, 6k would be the base pay for something that has no other modifiers than "they have 8 as their highest dice pool") then maybe it wouldn't be so hard to find work.
Plus it's always safe to assume PCs see a bit more action than most NPCs; probably because the NPCs have been around for multiple editions and the developers didn't want to make all of them thousands-of-karma unstoppables like the PC I've seen of the people who actually kept the same character through editions.
After a while you'll kind of need to slow down though. The players will likely have money and karma to spend that may very well take them over a week to use. Improving an attribute, increasing a Skill to Rating 5 or higher, and getting a Skill Specialization all take weeks. If you use the rules for making an Arcana test for Initiation (though I don't, personally) than that could take months to do as well.
I usually tend between two weeks to two months between runs; usually by looking at the payout and the events. If it was a pretty big run and some drek hit the fan, then usually the wait will be longer. If it is really quick and everyone comes out relatively unscathed, then it's a shorter wait. Because I figure, their fixer looks at them and goes "...You guys are fine, you'll be ready to make more money for me in only a few days!" Worth asking is if the videogames are any good examples; in
Shadowrun Returns: Dragonfall you are presented with what jobs are available and can choose whether or not to take them up whenever you want. In an actual game, I would definitely make it clear that the jobs have time constraints (nobody is willing to wait a month and a half for a team of runners) but otherwise, it's mostly up to the team to decide if they're ready to jump right into another run.