Associate Professor Brendan Zietsch from UQ's School of Psychology wrote an article for The Conversation discussing how long sex lasts which was featured in IFLScience.
The best study we have estimating the average time to ejaculation in the general population involved 500 couples from around the world timing themselves having sex over a four-week period – using a stopwatch.
That is as practically awkward as it sounds: participants pressed “start” at penile penetration and “stop” at ejaculation. You may note this could affect the mood somewhat, and might perhaps not exactly reflect the natural flow of things. But – science is rarely perfect, and this is the best we’ve got.