Edition does matter for this poll. When I run pre-4e games, my ratio is about 1 death : 3 sessions (I'm guessing).
But I'm voting per my current edition, which is 4e.
In my campaign in 4e, we've had 5 deaths total... in 84 games.
The lion's share of those were in a party with 2 or 3 leaders at a time though... since I started my new group, 3 of the 5 deaths have occurred, and it has only been running for less than half as many sessions as the O.G. group (20 so far).
I ran a 4e pickup game too, with about 5 sessions and about 6 deaths (but many caused by another pc).
Add to that the D&D Encounters stuff- I have run 3 weeks of it and killed 2 pcs.
So, for 4e, in 92 sessions I have 13 deaths, or roughly 1 death : 7 sessions.
Ideal for me is about 1 death : 3.5 sessions, I think.