Depends how hard you drive. As long as you dont drive with your foot down everywhere and bounce off the rev limiter it will last at last 20k. My original charger did 93k without a rebuild on normal pulleys.
I get mine done when its starts spitting oil out as mines a weekend car.
My current charger lasted nearly 8k but it was due to a bearing cage failing or something, pete knows as he found it while rebuilding it. Its done 6k since and it fine.
Before the cage failing i had the seals fail after i used some engine flush crap, wont be using that again. That had done 12k i think.
Depends on lots of things really.