About 30 is good for a softer compound aggressive tread. On lighter vehicles they last longer, but our WK's are pigs. I've got a set of BFG MT's from a Rubicon on mine and they are about done at 25 or so.
I've been through quite a few different sets of tires (sizes, brands, miles) on my wrangler. Rotating them is the most important thing. I rotate mine every oil change. They will last quite a while that way.
30k is about average depending on the brand, rotation, and weight of the vehicle.
See on my Wrangler with a solid axle, I actually got a lot more life out of some BFG Muds than I would have ever guessed. I bought them in 2000 for my first TJ, they were 32x11.50R15 and I ran them for about 20k on that one, then I sold that Wrangler and they went on the next one after a few years in storage. Then they went for another 30k on that wrangler and still had 5/32nds of tread left when I sold that wrangler.
It depends on a lot of factors, vehicle design, tire care, driving characteristics, tire design, etc. I have ran BFG's and only gotten 25k miles and also ran Cooper and managed 40k miles ( and those were mud terrain ).