“In my day I have prayed many such canonical hours myself, regrettably, and in such a manner that the psalm or the allotted time came to an end before I even realized whether I was at the beginning or in the middle. … But, praise God, it is now clear to me that a person who forgets what he has said has not prayed well. In a good prayer one fully remembers every word and thought from the beginning to the end of the prayer.” 

Luther, Martin. “A Simple Way to Pray” (1535) Luther’s Works Vol. 43. Philadelphia: Fortress Press, 1969, page 199.

Comment: Our minds wander all the time. Even when we are in prayer or at worship or during a sermon. (Try especially not to let that happen when you are leading prayers and worship, or preaching!) In prayer times, if you get to the end – however you are measuring that – and can’t really remember what you said, there is good reason to question just what you’ve been doing. This may well mean that you shouldn’t pray for too long a time. It could be like advice for study or exercise: repeated short periods of time are usually more helpful than long stretches.