You are thinking the wrong way around. The reason google like more content is that it is good for visitors. You should add it because there is more information available to be added about a topic, not because you feel you have to because of rankings. I don't think it would make a difference between once a week and once a day.
Google and other search engines like you to make your website for humans and updated regularly. Do not add content just for them, do it for the people. ~ Mike
The more you update your site content the more visitors will come back to visit your site, and always try to do what is really helpful for your website visitors then search engines will follow
If it's a static site, you should add at least every 2 months, otherwise expect the traffic to eventually die away. It will take google about 6 months to start delivering less traffic to you. If you have a service so the front page doesn't change much, add a blog and link to the blog's latest posts on the front page. Easy way to do this is with http://feed.informer.com/
Do you know how to create nice rss feeds withoud feedinformer? I don't like it cos it requires a link back to their site.
If you are after pr then you should update your site at least once a week but if you are looking for some visitors then you be updating it daily.
If you don't like to link to other working people, then you can create one by hand. It's easy - just needs lots of time to create your own RSS feed - including submission to all RSS directories. Once done however you get backlinks PLUS traffic PLUS changing page-content if you SSI the feed into a few pages of your own site.
Is there are written rule that one should update the site atleast once a week for a gain in PR? PR is updated once in 3-4 months. Personally, I feel that we should update the site as and when it requires. Say for example, a news site is updated on a hourly basis. A trading site is updated on a minute (if not seconds) basis.