Web content is not something written in stone since browsers are constantly evolving so there is no assurance that some content created in present time will be rendered by it the same way in future - i.e. at some point it will look and behave differently.
Therefore, it should be checked and updated periodically - on WordPress sites this means updating theme and plugins, checking if anything is broken in the process and then repairing it.
After the site is created and published, I will give You a price for yearly maintenance (first year is free of charge). It is typically about 40€, depending on number of plugins and site complexity.
This includes above stated actions every 3 months after site is published after which I create a backup and store it on my high quality HDD.
This is something I highly recommend but isn't mandatory. If you decide not to pay for maintenance, i give You no guarantee on future site behaviour and take no responsibility for it.
I can also be hired to do other actions for a prepaid monthly/yearly fee.