How accurate are read receipts?

On iMessage , when. A person has read the receipt how accurate is it for when they actually read it ?

Also when the speech bubble appears , if somebody doesn't click off it , for example types "hello" and forgets to send it, would the speech bubble permanently stay there or would it disappear? I've had one experience of seeing a person speech bubble on for ages and they haven't been "texting" at that point.

Please advise thanks