John Gill Commentary 1 Kings 11:16

"(for Joab and all Israel remained there six months, until he had cut off every male in Edom);" — 1 Kings 11:16 (ASV)

For six months did Joab remain with all Israel
With the whole army:

until he had cut off every male in Edom ;
as he supposed; for it was not fact, since after this they increased again, and became a powerful people, and had a king over them, and revolted from Judah, (2 Kings 8:20–22) .