First, an aisde: I'm truly surprised at the number of posts here indicating that character deaths are not tracked or recorded. I just can't imagine running a game and not keeping *some* records, the most important of which would be:
- new characters (name, race, class, player)
- character deaths
- character changes (retirements, race/class/name etc. changes)
- adventures played
- brief notes on who did what when, if only to give out ExP later
Now, to the main question:
While they tend to come in batches, my current campaign is running close to 1:1.5 deaths vs. sessions: 105 deaths in 176 sessions. That includes party NPCs, who I log the same as PCs; to sort those out would take longer than I'm going to give this.

But the vast majority of those deaths were actual PCs.
The other campaigns I've run have been closer to about 1:3 at a very rough guess; but they've also reached higher levels and - truth be told - have at times had much more cautious players at the table. My current crew are a wonderfully gonzo lot...
As for what I prefer: I like some turnover, but prefer it to be steady rather than in batches: batches are annoying. My Friday group, for example, got beat to shreds (lost 4 PCs of a party of 7) in their previous session, then went to town last session and recruited a whole new set of victi...er, members; so 5 of 8 are now brand new and I need a bloody program to tell who's who! Fortunately, this is one of the more excessive and rare examples...
I find that once the PC Clerics get to 9th (or can cast 5th level spells) and thus can Raise in the field, there is very little character turnover...sure I can kill 'em, but they just come back. After that, in the long run things tend to get a bit too static for my liking.
I suppose my preferred rate of turnover (for whatever reason, not just death) would probably involve swapping out most of the party over the course of a year or so; more frequent if people have a stable of PCs they swap in and out from depending on what the party needs. I also prefer multi-party campaigns, to promote said swapping; and do my best to run such when I can. Linear or single-party campaigns tend to be much more static.
Lanefan