"An erection means that the penis turns hard and increases in size".
Peter Mayle.

"Erection is a cause of sexual arousal, for instance when you stroke your penis, when you look at the breasts of a young woman, when you imagine kissing someone or being naked with someone. Sometimes it happens in the most inappropriate moments. Some young men get an erection as a result of anxiety, for instance while standing in front of the whole class or while talking to a woman. It is also very common that men awake with an erection in the morning or when they have to urinate. Erections can happen unexpectedly and for no reason".
Karen Gravelle.

"Between a few seconds and half an hour. The muscles that surround the base of the penis will gradually relax and blood will continue to flow normally. Then the penis will soften and return to its normal size. This happens either just like that or after an ejaculation".
Karen Gravelle.