You don't have to update everyday - you just need to try to keep the content fresh. Adding new content once a week can work as well if you're high up there in the serps and rank good.
if by update you mean the skins? i do it everytime the season changes. for content everytime i get bored. its up to you.
to quote a cliche "Content is King", unique content is what raises your serp's and the more you have the better!
It completely depends on the subject. I have some sites that are updated daily, some only a few times a year. For example, if you have a site about Christopher Columbus, chances are you aren't going to be updating it every day; if you have a site about Britney Spears, you probably want to update it more often.
As long as you keep your content up to date that's fine. Always write for your readers and not for search engines...
Depends if its a blog and you want people to actually come back I would say once or a couple times a day. If its something you dont care about return people then once a week, or whenever you feel the need. Like said above...Content is king. The more info you get out there the more people will be able to find....your odds are better if you have 10 posts in 2,000,000 than 1 post in 2,000,000 haha 10x times more likely to be found...but thats just my thinking on it...
My top producing web site has not been updated in over a year. One concern I have is, if I add new content, will I not get the traffic due to content change? And then I have others where I have been adding fresh content on a regular basis and little traffic. Both domains are over 7 years old so there are not new.
It's actually something of a myth that all your sites need to have content updated regularly. Google does like fresh content so it's a good idea + more content = more search terms your site is found for. Updating once a month would be fine once established. Dean
You update the contents for your users/readers, If the contents are good, they will re-visit your site. Do you have to update your contents every minutes to make Search Engine Bots happy? NO you don't and you don't have times for that.