I have heard that some people take as little as 15 minutes to pray 5 decades of the rosary. In order for this to be possible you have to say it very fast, like with no pauses at all.
I have timed myself and if I pray it at a "normal" or "regular" pace, not rushed and not slow either, and with no interruptions, I take 4 minutes and 30 seconds on average for 1 decade, and I only enunciate the mystery, say 1 Pater, 10 Aves, Gloria and the Fatima prayer, and that's it. I don't add anything else to each decade. I know some people may pray it according to the various methods of St. Louis Montfort, and that makes it even longer.
Even if I were to round it to 4 minutes for each decade, that would still be, at the very least, 20 minutes for each mystery.
So how can some people take as little as 15 minutes? Although I don't use any of the methods of St. Louis, since they are counselled and not commanded, he did say it's bad to pray it fast and without any pauses, and I don't think that was a mere counsel.