I voted 10K, which some of you may find bizarre. However, there are folk out there that cover less than 2,500 miles a year, and so tyres that lasted 20K would be 8 years old by the time they should legally be replaced, yet these could be perished, cracked and downright dangerous.
I got about 18K front and rear on Yokohama s.drive tyres, but that was less than a year's worth of driving. It's odd because these tyres are really grippy, so I expected them not to last this long. The Yokohama A539s I had previously were less grippy, yet were toasted in less than 10K.
On a side note, why do you have to replace brake pads and discs in pairs, yet tyres you can do what you like? It is legal to have remoulds on one side with only 1.7mm, and brand new Pirelli P6000s on the other with 7mm of grip. Kind of like marrying your cousin, it's legal, but not right.