The sweet spot that I've noticed seems to be around 3 years average. Anything more than that doesn't appear to make a difference in overall happiness levels. Lower than that and I start to have concerns.

This was a question I used to ask my consulting clients to get a feel for what sort of department I was going to be walking in to. The higher turnover places always had the worst development practices in place (or none at all, like a complete free-for-all), and the places with less developer turnover tended to have fewer people focused on guaranteeing themselves job security & more people focused on efficiency.