The average man can stay erect during sex for around 5 to 6 minutes before the erection is lost.
Then it might take awhile for the man to become erect again after that.
So if the man is erect the sex should begin within a minute of being erect otherwise the erection may go away too son and you won't be able to have sex until the man becomes erect again.
You can take some erection medication that increases your time of an erection but be careful when taking those erection pills.