Loading time and page loader display (when to show)

I haven't been able to find answer to my question from the internet, so maybe someone knows how to help me.

What is the minimum waiting time after when should a page loader be displayed to the user?

For example, I have a situation where the page loads quite quickly, but there is a rule that page loader should be displayed no matter what, so as the loading time is short, the loader only blinks/flashes. Now I'd like to know what is the best practice for a rule "show the page loader only when loading time takes more than ... seconds".

By a page loader I mean this thing:

Thank you!