Focus Timer is a simple and minimalistic app that helps you concentrate.
Just set the desired time and press Go to temporarily disable notifications.
It will automatically enable the notifications again when the timer is finished, even when your screen is locked or if you're working in another app.
You can disable it before the timer finishes by pressing the Stop button.

If you open the app for the first time, then it will guide you to the settings of your phone. Please make sure that you give permission for the Do Not Disturb setting.

The app is completely free to use, so don't hesitate and time your focus!