Is this irritating or tiresome for you? If so (and assuming it does not stay erect after ejaculation), presumably you can get it to go down by causing yourself to ejaculate? It's not entirely surprising that a guy could get an erection that lasts a long time if he doesn't do anything to make it go down.
The length of time you have erections is really nobody's business (besides your own) unless it is troubling you or hurting, and then it becomes the business of your doctor. If it is a problem, and if ejaculating doesn't relieve them, see your doc.
Unless you are having intercourse, that sounds uncomfortable and a lot of women would not be thrilled with a 45 minute erection if they are being honest. Life is not really a porno. This common or a one time thing for you?