Just two days ago I boosted a pair of refer-a-friend 21's up to 35th in SM: Cathedral. (Took 20 minutes per run, they obtained one full level of xp per run until 30+, then it slowed down a bit.)

At 35th I portalled them to Shattrath to hearth, then portalled them to Stonard and hit Sunken Temple. (After 1.3 runs they are now high 36th.)

About 6.5 hours total to level 15.5 times.