I've only seen seven episodes so far, but based on that I would expect the tenure of individual team members to be measured in months, if not weeks. They're a small team with no support; there is no option for somebody to stay on the bench while they recover. There's precious little cameraderie or team unity, either. They are constantly dealing with not just extreme physical danger but also the kind of emotional trauma that some people never recover from even when it only happens once, and they're getting hammered with it every couple of days. Unlike Dr. Who, there is no sense of downtime for these people, either — you get the feeling that if they go home at all, it's just long enough to get not enough sleep before jumping back into the fray.
In short, they're running in high gear all the time, in situations where the odds are against them. Adventure heroes can get away with that and look good doing it, but Torchwood is really more in the horror camp, and trying to play it realistic. There's only so many situations you can survive before the odds catch up to you, and the team members which aren't killed outright should be having mental breakdowns left and right. I just don't see how these characters can continue to function through three seasons of story.