I had a great conversation with another small business owner the other day. She runs a small design firm that he wants to stay small and nimble. She encourages turnover because more senior employees demand higher salaries and better benefits, and upsets the overall balance of power for such a small firm. So 3-4 years is the longest they let anyone stay, unless they're on a partnership track (which one staffer currently is).

Obviously the dynamic with is different for hackers, and especially those working on a single product over time, but it was a refreshing perspective.