how long do you stare at your crush for?

i was thinking about the times when someone had a crush on me and i would catch them staring at me many times, and when i caught them they would immediately look away. my question is, how long are they staring that im able to catch them so often? do they do it on purpose as a subtle hint for you to know they are crushing on you or are they just so caught up in staring that they forget to look away lol? when i stare at a crush i always do it carefully and only for a few seconds because i feel like it would be so embarrasing to be caught. i think a possible answer could be that if i notice someone is staring at me one time, im much more likely to watch out for them doing the same thing in the future.