How to modify the length of the post summary

Hello guys,

I would like to cut by myself where the summary of the post should stop and then readers to click to Continue reading.
Any idea how to do it?

Here is it how it looks like at the moment

Thank you

EDIT: Sorry, i got it, just need to add ===
But I put it after a text with no line and it basically change the typo instead of creating a delimiter.
Thanks anyway