Keeping notifications permanent [duplicate]
Solution 1:
Unfortunately, the notification timeout cannot be changed. It is actually not a fixed timeout. "if you move the mouse up towards the notification, the timeout is prolonged, and notifications do not hide when hovered."
However, it is possible to disable Notification popups. This way, it won't disrupt you, but it still remains available in the notification area you see when clicking the date or by pressing Super+m.
To disable notification popups, head to "Settings" - "Notification".
Solution 2:
https://extensions.gnome.org/extension/3795/notification-timeout/
This extension worked perfectly for me trying to tackle the same problem. Apart fromthis, there seems to be no other way in setting or dconf.