Behavioral mirroring is often a sign of trust or comfort, and applies to levels of eye contact as well. If someone is subconsciously doing the same things you are doing (e.g. longer and more direct eye contact if you initiate, more or slower blinking, more smiling, more relaxed postures to mirror yours), those are good signs of interest.

The converse is also true. If you are doing a lot of direct eye contact, and it seems like the other person is often looking away or closing up their body (crossed arms or rotating their torso away), that’s a sign to reduce some of those behavioral signals to match.

I’d say there isn’t a “This exact amount” to most things, as people are all different in their preferences, and it’s more about adjusting up and down with someone, in response to their small non-verbal or body language signals. They will likely be doing the same with you. Also, as others have mentioned - you can be more direct with words. If this is something you’re unfamiliar with and there’s someone you trust, you can say directly that you’re uncertain and ask something like “I’m not great at knowing how much eye contact feels correct, could you let me know if you notice too much or too little?”. If they are friendly with you, they’ll also likely be comfortable with the small request.

Also, just to say it - eye contact can mean the general eye area - it doesn’t mean your exact pupil to their exact pupil. I find that if I focus on the literal eye/pupil, then I get strained trying and keep attention on that specific small area. If I focus on the general eye area (nose/forehead/eyebrow/general eye) - they both can’t tell that it’s indirect eye contact and it’s easier to let my body auto pilot focus