Sure you can install the OS X El Capitan update when you about to dash off to a meeting, or click install right before friends drop over, but it's not a great idea.
Although small, there is a potential for problems with the upgrade process and you do need to pay some attention to the update to install it and get it up and running. This is easier to do if you aren't distracted by something or someone else.
We suggest setting aside at least 30-60 minutes to download a large OS X El Capitan update file and then you may need to spend 30 to 60 minutes for the actual upgrade. You can use your computer during the download, but not during the upgrade.
Plan to spend at least 15 minutes for a small OS X 10.11.4 or similar update. The download is often fast, but you will need to spend time waiting for the computer to install and reboot -- during which you cannot use your Mac.