I think that you can go a long time, even forever, without being hit. I was a messenger in Phila. and got hit a total of 3 times, once my fault, once by a hit-and-run, once by a guy that did it on purpose. Since I've left that job, I have not been hit, still ride to work everyday plus 150-200 + miles (200-300 km) per week training for road and track. So riding around in the densest traffic in your city (mine is Philadelphia) will increase your odds of getting hit, also if you spend all day on a bike in the downtown areas, you increase your chances. Even so, I am sure I could have avoided each of the three incidents I have had. The hit and run driver ran a red light, she was fully culpable, but knowing how people drive in this town (crazily), I could have been a little more careful.
Bottom line: Ride defensively and maintain high visibility: Wear bright colors, reflector and lights at night, HELMET always, and don't ride an inch (2.5 cm) from the curb (kerb), get out in the lanes a little so people can see you and have to wait til it's safe to pass you. I think that a lot of people get into dangerous positions by cramming over as close to the side of the road as possible; leave 18" (0,5 m) or more between you and the road's edge.
Enjoy!