Assuming you mean Exalted, it takes quite a while..
If it's worth grinding depends if you want the mount or not
If you do it in a group and kill stuff fast you should be able to make somewhere between 1000 to 1500 rep per hour and you need 42k if you haven't gotten any rep yet. You also get 250rep from doing the dailyquest 'Path of the mistwalker'
Oh I read that completely wrong :P
Guessing you mean black prince then? In a group 2 hours tops if you start from 0, personally I did friendly to exalted in 1h10m
Thunder Isle, as in Sunreavers, or Shado Pan Assault? Sunreavers can be done in a few days, Shado Pan Assault involves clearing ToT weekly for a a month or two.