Oliver Cowdery initially omitted the that after “from the time” in this passage, but later he supralinearly inserted it with somewhat heavier ink flow. Including this example, there are 18 examples of this expression referring to Lehi and his party leaving Jerusalem (for the list, see 2 Nephi 1:24). Of those 18 examples, 11 have the that and 7 do not, so either reading is possible. There would have been no strong motivation for Oliver to have emended the text here in Jacob 1:1. Instead, the correction is probably the result of Oliver’s proofing of 𝓟 against 𝓞.
Summary: Accept in Jacob 1:1 Oliver Cowdery’s addition of that in the printer’s manuscript (“from the time that Lehi left Jerusalem”); the original manuscript (not extant here) probably had the that.