Mormon had the time between the years 362 and 375 (about 13 years) in which he "did utterly refuse" to fight, and instead "stood as an idle witness." In addition, at least the last seven of those 13 years that he refused to fight (367-375 A.S.) were peaceful between the Nephites and the Lamanites. Thus, Mormon had peaceful access to all the records in the hill Shim until the year 375 A.S. when he took all the sacred records from the hill Shim and retreated from the advancing Lamanite armies (Mormon 4:23). I don't think we can know when Mormon started his abridgement, but the phrases in Mormon 5:8,9,13 seem to be referring to that process in a time period just after Mormon had 13 years to devote himself to a complete understanding of the Nephite records:
"But, I, knowing that these things must surely be made known, and that all things which are hid must be revealed upon the house-tops--" (Mormon 5:8)
"And also that a knowledge of these things must come unto the remnant of these people, and also unto the Gentiles . . . therefore I write a small abridgment . . . " (Mormon 5:9)
"And this is the commandment which I have received . . . " (Mormon 5:13)
In the year 375 A.S., Mormon returned to his command of the Nephite armies (Mormon 5:1). After a five year period of Lamanite devastation upon his people, and a four year period of gathering, Mormon makes the statement:
Knowing it to be the last struggle of my people, and having been commanded of the Lord that I should not suffer the records which had been handed down by our fathers, which were sacred, to fall into the hands of the Lamanites, (for the Lamanites would destroy them) therefore I made this record out of the plates of Nephi. (Mormon 6:6)
It is hard to tell by Mormon's phrase "I made this record" whether he was just completing a record already started (maybe during his 13 year interval of standing as an "idle witness") or whether Mormon's 13 year period of standing as an "idle witness" allowed the time for him to sufficiently acquaint himself with the Nephite records that he was able to make his abridgment after he had gathered in his people and before the final battles (Mormon 6:6). [Alan C. Miner, Personal Notes]