4 years has been my experience. Many have been around longer, many last much less but 4 years seems to be an average. I was laid off at the end of last year from a 91 operator who had the same Challenger for 17 years (sadly I was only there for 4 years) ...money got tight and the airplane went away. I am thinking airline now, there is little security in 91 flying...I love it and have been doing it nearly 20 years, time for more stability.
edit- I'm the new guy here, actually came for some airline info but saw the corporate forum and had to drop in.