I hit the jackpot twice myself. Once was about 2-3 days after they announced it, and it happened again a few days after that. The second time I got it, I had actually just quit out of a game, so I don't think quitting has any impact on the probability of you hitting the jackpot. I think quitting only affects your chances in the sense that you won't receive your payout if you leave early. It would be like playing the slots at a casino and walking away before you found out the results.

E: Well isn't that ironic...I just got a credit jackpot of 3000 cR in my last game of Firefight. Netted 23733 cR for that game.