I would think trying to encompass all of a person's memories with a single spell allow you to remember it, would be something akin to a minor ritual spell, even possibly taking on a number of magnitudes simply for the complexity.
As Kid Gloves said in True Lineages they give three base levels for different levels of retrieving memories, unfortunately they are all Creo magics since it seems that retaining a memory falls under the purview of making a thing whole and complete.
Base 4 Restore a memory of a brief event to a fresh state, as long as a fragment of it remains. The affected memory can be no more extensive than a short conversation (two or three rounds).
Base 5 Restore a memory of an event to a fresh state, as long as a fragment of it remains. The affected memory can be no more extensive than about two minutes.
Base 10 Restore a memory of a day’s events to a fresh state, as long as a fragment of it remains. Events are remembered as if they had occurred only an hour before.
Also, discounting the ability, Art of Memory, may be quite a oversight if your maga truly wishes to remember great swathes of someone's life. First off it is a common ability among the scholars of Europe and as such there are no special requirements in attaining it, other than possibly needing a text or a teacher. Second, without the use of a spell the magi can remember enormous amounts of information with the benefit of it being perfect and exact in details with no flaws.
So then you would not even bother with things that might affect your spell roll you simply roll add Intelligence and Art of Mem score and you get complete recall. Easy and awesome.
Also in the Mysteries on page 27 there is a spell using Rego Mentem base 5, that reinforces the memory palace against magical attacks or attempts to modify memories. This would seem to indicate a sort of ward against Mentem magic ( I wish my thoughts were clearer at the time of this writing 
) but I think you could possibly argue that using a Base 5 Rego Mentem spell would allow you to hold a precise memory in your mind for the length of the spell.
Perhaps using the bases of the Creo versions to make Rego spells.
Base 4 for a conversation
Base 5 for a two minute segment
Base 10 for a entire Sun amount
using this one may wonder if
Base 15 for a Moon/month worth of memories?
Base 20 for a Year... ritual of course
Base 30 for permanently placing the memory in your head? Should cause warping I would imagine.
Perhaps using these guidelines you could argue that the spell simply keeps the memory rigidly in place, protecting it from any outside influence?
Ohh... make a memory device powered by spirits to maintain the target's memories forever within the device.... ohhh.... project! I call dibs.
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