As mentioned its how you drive and how much TQ you got... my CL65 tore thru my PS2's but it had almost 800 lb/ft of TQ.
My 997TT has 12k miles and the PS2's are almost done. I think 10-15k is fair based on driving habits and TQ.
2013 Mercedez Benz SL63 AMG Diamond White
2013 Porsche 991 Carrera 4S Carrera White
2013 BMW M6 Alpine White
2012 Audi R8 V-10 Ibis White
2012 Mercedes Benz C63 AMG Black Series Obsidian Black