30 K miles from either the NT-555 or the NT-555 G2. If buying new, get the G2 version.
if you want really long life the Nitto MOTIVO has a tread wear rating of 560. I use Motivo's for fall + winter, and NT-555's for spring + summer. ( I have 2 sets of rims, so 8 in all).
http://www.nittotire.com/passenger-tires/nt555-g2-ultra-high-performance-tire/
They make the motivo in a 255-35-20. The motivo has better traction than my NT-555 in the dry..and WAY better in the wet. The new 555 G2 is a different story. The g2 has slightly better dry traction than the motivo, but the G2 is not quite as good in the rain vs the motivo.
On paper, the motivo should be good for 56 K miles.