The only limit is your computer.My finals are always at 300 dpi with a size from 3000 X 2000 to 5000 X 3000 pixels and sometimes bigger.The rendering time variates very much depending on the effects you're using,the ammount of polygons and the number of objects in the scene.In case of my image "Nur f Papier !(papers only !)" the final rendering took 3,5 hours,but for "Hof 2" it took me 48 (!!) hours to render it (final quality).So you have to be patient sometimes,but it's worth it !!

http://www.fredivoss.de 

...yippi ah yeah or something like that...