In your opinion, how common is jealousy as part of a healthy relationship and at what point is the exact limit where it becomes too much?

Not asking as I’m experiencing it, only asking as people have different ideas of what is ‘too much’ with jealousy. Usually looking at other Partner’s phones etc but do you have any ideas?
In your opinion, how common is jealousy as part of a healthy relationship and at what point is the exact limit where it becomes too much?
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