on average, starting from the day you toss the returning disc in the mailbox, how many days pass before the next one shows up at your house.

also, how many discs/titles do you think you watch in a month?

Superfast for me. Usually 3-4 days.

If I return something on Tuesday, the next one pretty much always shows up on Friday. So, a three day turnaround.

2 days each way, so 4 days

Usually 3 days, sometimes 4.
If I stick it in my mailbox on Saturday, I will usually get a new disk Tuesday or Wednesday.
Dropped one in on Monday, supposed to get its replacement tomorrow, might get it today.
I actually had a big excel file where I tracked 'date returned' date accepted date sent and date recieved, and over the past two years, there was a marked improvement [with some rare exceptions, of course.]
Then again, I'm only about 90 miles from a distro center, so that helps.
I don't watch as many as I could, I've slowed down a lot, but I'm trying to get my money's worth again [I'm on the two out plan.]

My turnaround is three days total. So if i mail Monday, I get the new discs on Wednesday. Occationally they take an extra day to check in the return but that's an exception.

Same here.

From March through late September I'd been getting a two day turnaround 95% of the time - that is, I'd mail a disc in Monday, they'd receive it Tuesday, ship a new one Tuesday, and I'd receive that on Wednesday. For the past month, however, 90% of my discs have taken an extra day to ship. At first i chalked it up to fluke/coincidence, but it's obviously become a pattern now. I'd wish they'd keep their four bucks and go back to mailing new discs the same day they receive old ones.

Has this been happining to other people? I'd like to know if it's systemwide, just my region, just me, or what.

They recently opened a shipping center in my town so my turnaround has become screaming fast.

2 day turnaround, drop it in the mail monday, new disc on wednesday

To your 2nd question: I average about 3 per week or 12-14 per month.