5 years would be the average at the moment. As short as 1 year for a few guys recently who have taken a 737 base.
SO is a bloody great job, if 6-8 years out of a career of 30 odd years scares you, then you probably won't suit the job. Yeah you do get to know a few of the frequencies and how to say hello/goodbye in Russian and Indian but who cares, most of us love it!