It really depends on your website. Some sites have content that needs to be continually changed and updated, because it is time-limited and if you leave it up there for longer then it will become irrelevant or, worse, incorrect. Some websites don't need updating very often, because the content isn't changing and remains relevant for months or years.
If you want your site to grow and flourish, and particularly if you want to develop a following and interact with your regular visitors, you need to be looking at ways to add new content regularly. How regularly? That depends ... how regularly can you keep adding high-quality and worthwhile content to it? If you can put something up there every day, that's fantastic – if you don't have the time or resources to post new stuff more than once a month then once a month is fine, but try to pick a schedule and stick to it, so that people coming to your site know what to expect.
Don't force yourself to post new content just for the sake of it, because if you do that then you're likely to end up churning out rubbish that people won't read, and search engines won't care about either. There's no set frequency that is required, it's whatever works best for you and your website.