You can of course pause a render,save it and render it from where you stopped later (like when you are sleeping) as long as you don't use "render to disk". A lot of the time anti-aliasing is what takes the longest and that isn't taken into account in the render time estimates. I had my "Seadragon's Realm" render in like 10 hours but it took 5 days for it to AA (on a 900 Mhz machine) Some people have said that they created scenes that would take years to render. :)