I have Kenda Small Block Eights (35 mm) that are coming up on 3000 miles. The tread pattern is almost completely worn off the centreline of the rear tire, but the front has plenty of wear left. I'll keep running them until I start to see cords on the rear or start getting punctures (got the first puncture for either tire on the rear a couple weeks ago--a sharp stone was the culprit that time). I don't expect they'll make it to 4000 miles.

The 35s are a bit iffy on fender clearance so I'll go to 32s or maybe even 28s depending on what's in stock and reasonably priced when I finally get around to tire shopping.