When you call an entity into the scrying mirror

does the entity go in and out showing its face every few minutes in the mirror through out the magickal working or does the entity remain there constant until the spell or “astral evocation” is over?

The image stays as long as you are able to hold it. get out of sync, lose concentration, doubt yourself and the image can vanish until you can get back in the zone again. any kind of single pointed awareness meditation can help discipline the mind to stay focused and quiet.

For the last time: the spirit is not in the mirror. It doesn’t even appear ‘in’ the mirror, because when it does, the mirror will have vanished from sight. You don’t even need the mirror in principle.