What’s important to remember is that a website is more of a service than a product. It’s not enough to “deliver” a website to a user. You should also make sure that everything works fine, and everybody is satisfied and always be prepared to make changes in another case.
Feedback system added to the site will allow you to detect possible problems the end-users face. The highest priority task, in this case, is to fix the problem as fast as you can. If you won’t, you may find one day that your users prefer to use another website rather than put up with the inconvenience.
The other important thing is keeping your website up to date. If you use a CMS, regular updates will prevent you from bugs and decrease security risks.
Estimated time: ongoing