That's about the experience I get as well.
A good mix of content, some spun, some unique and some plain copied gets me success most of the time.
It's part of the "churn and burn" style of IM. I have expectations of the average site lasting perhaps 9-18 months as an average. I've had some picked off by Google inside 6 weeks - I've had others earning using mostly none unique (but very readable) content for many years.
In churn and burn it's a bit of a lottery. But the set up costs are so cheap that you can afford to keep throwing another hook in the water.
In my experience - going through my hosting accounts a few weeks ago. Looking at the domains I've allowed to lapse because they were Google bombed. That 9 month average seems about right. Every so often you get a gem though. I have a site for golf holidays in Portugal I put up in 2007 that has never had a single unique word put on it and makes a tidy sum month after month and has been for 8 of the last 9 years. I've had a couple of others that didn't make it 6 weeks.
If I could accurately tell what difference there was between those two sites or the markets they were in - I'd be a millionaire inside 18 months.
But every time I think I have cracked it, worked out what short cuts work and what ones don't. I'm confounded by a crap site doing really well, or a good site bombing early. But at $50 to set up completely (site, hosting, theme and content) for my churn and burn sites, I'll just keep rolling them out and accepting the overall good returns I get.