I did Senegal and back (on a 1200GSA) which was 9000 miles on one set. At low speed the wear rate is low, it's the higher speed motorway sections that hits the treads.
I put the TKC80s on my F650GS twin before the last Morocco trip and they have been on for just over 6000 miles. There's still a lot of tread left but not enough for the upcoming Morocco tour, so I'll have to change them ahead of time.
Tim