http://rantsandflames.com/xpcalc.php
From my own personal anecdote, I averaged a level about every 2-3 hours from 17-50, sometimes faster in the right spot and sometimes slower. In the 50's it slowed down dramatically.
As an Iksar Necro you have some additional exp penalty to contend with, but I suspect you should land pretty close to the same benchmark in the right solo spots.