What happens to my Google rankings when my site goes down for a short or long period of time?
Google does take into account up-time and downtime, it also monitors response time. Your listings would not be deleted for a short downtime like this however if you rely on popular keywords your sites rank might fall depending on how competitive the category you are in is.
Here is an article that explains the position that Google is thought to take: http://jamesmartell.com/matt-cutts/can-a-sites-downtime-affect-its-ranking/
You should expect your rankings to be affected when your site goes down for a lengthy amount of time. Normally you're find that a site will drop rankings as a temporary effect and can regain the authority it lost in good time. However if your site drops from Google's index then this situation is a lot worse, depending on your authority on your site will determine how quickly you re-rank your website.
Google can take days to weeks to remove a site from it's index and generally will remove smaller sites a lot quicker than larger ones. No one can tell you exactly how much rankings you're lose until you try it for yourself.
I think a short downtime will not be a disaster for you, unless you're dealing with some of the more timely queries like news, celebrity gossip, the precise location of bin Laden's toothbrush &c… or some of the more popular queries / competitive search niches. Or unless your site is so big and important and it's getting crawled often enough that an eight hour downtime means the spider has returned two or three times to continue finding nothing there.