There are a couple of elements to track when it comes to keeping your website updated:
First is the technology. WordPress is constantly releasing updates of itself, as is true for any plugins you use, and potentially even your theme. Think of it just like the apps on your phone — just as your phone will let you know when updates are available, WordPress will too. And you want to stay on top of the updates because they often fix bugs, security issues, and add new (desirable) features. Not keeping your WordPress installation updated may result in an increase in the likelihood of your site being hacked by those people intent on doing such things, and no one wants to deal with that.
Second is your content. Ideally, you’ve identified when coming up with your site content in the very beginning what you plan to update frequently. The idea being that if you’ve planned for the most frequently updated items, you can design and build your site to make sure the focus is on the ease-of-use for updating those elements. For example: If you really want to be able to update a featured background image seasonally, then make sure it’s created to allow you to do that easily. Then, create a schedule for yourself to check in on your site to keep the content fresh and relevant. Whether it’s weekly, monthly, quarterly doesn’t matter as much as your commitment to doing it. You’ll benefit from additional searchability by adding new content, and you want your site visitors to know you’re actively invested in the project by keeping it fresh.
Think of your website as a project that’s never finished, as opposed to something you create and then leave. It’s more like a garden, and less like a chair.