I think there are a lot of variables like when was the last cum, how horny the person is, type and quantity of lube, things like that.
I know sometimes when I'm watching a really hot porn video, I'll shoot my load much quicker than 4.5 mins (even though I'm much older than 25 now still applied when I was that age). Other times it took a long time to cum, especially if I didn't have a whole lot of lube or I was outside in the cold trying to jack out a load..