How to change time till screen darkens?

Hello.

When are not using your phone for a while, the screen automatically turns off. (In about 15 seconds).
How do I change this time? I can't find the setting.
And how can I make it so that momentarily it doesn't turn off?

PS: I have the Xiaomi.eu on a Mi8 but I suppose it will be the same on any MIUI.