Here Oliver Cowdery initially wrote “for many days” in 𝓟. As David Calabro points out (personal communication), Oliver was probably influenced by the use of days at the beginning of this verse: “after many days their dead bodies were heaped up upon the face of the earth”. Almost immediately Oliver crossed out the second days and supralinearly inserted years (there is no change in the level of ink flow). Either reading is theoretically possible. Elsewhere in the text, there are five occurrences of “for many days” and four more of “for many years”. We get similar variety with the expanded phrases “for the space of many days” and “for the space of many years” (with 11 and 8 occurrences respectively). There doesn’t seem to be any strong motivation for Oliver to have emended days to years here except that 𝓞 read that way. The city of Ammonihah was rebuilt some years later (Alma 49:3), over eight years later according to the chronology specified in the text (compare Alma 16:1 with Alma 49:1). Thus years is fully appropriate here in Alma 16:11, although days would not be impossible. The critical text will assume that the original manuscript read years.
Summary: Accept Oliver Cowdery’s corrected reading in 𝓟, “for many years”, rather than what he initially wrote, “for many days”.