It is advisable to spend an hour when you can in private adoration, also.
Fifteen minutes is fine when praying the prayers and contemplating the words; more time than that, is usual, if one were to say the prayers with ‘prolonged’ meditation on the Holy Mysteries. But there are no hard and fast rules, except for the guidelines set out as applicable for certain devotions.
Some people say all five decades, and some pray a decade. I think to pray all five are best. But if only time for one decade, go for it.
As said, in some devotions, time plays an important part e.g:- First Saturdays specify minimum time requirement for the prayers. Time, playing a part, because, outward expression is a part of our faith, as human beings alive in a temporal existence. However, if one accidently spent fourteen minutes praying, instead of the required minimum, I would expect that one’s initial intention, to pray for the minimum amount of time, would be taken into consideration. If you forgot and could only pray for one minute then one would presumably offer that up. So, time can play a part.
Comparing power, is not necessary, as all is in God, for God. Our Lady’s intercession is present, for the Church, whether we are in attendance at Adoration, or praying the Rosary.