Where do the sith go when they die?

I'd say that it all depends on the Force practitioner and their level of skill. I'm pretty sure I remember reading somewhere that Obi-Wan could only manifest for about 10 years after his death... Qui-Gon didn't know how to manifest himself visually, but he was able to communicate with Master Yoda over 10 years after his death... Exar Kun was able to manifest himself several millenia after his death...
I wonder if Darth Maul haunts the Theed Palace Reactor Room? ;D