They usually use a mixture of rendering software, a lot of which will be in-house software. Although they may use package render systems like 'Auto-desk Maya'.
I know from being an obsessive freak (Joke at my expense :P) that Dreamworks, during the production of Shrek 2 had a server farm of about... 700 servers (I think that number may be wrong, worth looking up) going almost 24/7 for the entire production time.
So, for the rendering of UP, try looking at how many years it took to make the film. minus 1 to 2 moths, and youll have approximately the render time.
BUT - Its complex! You have to remember that a lot of the renders don't get used in the film, and a majority of renders are just test renders, are you counting those too?
For further information try getting hold of a 2 disk version of Up, and seeing if there is a Behind the Scenes section. There should be.
I haven't been able to answer your question as such, but I hope in some way I did help you.