I posted before about guys who came back when, after ACL surgery, and how the average for an athlete is 9 months (about 39 weeks). Which would mean Mahomes would be playing maybe week 2. Adrian Peterson came back after some 32 weeks and had a fantastic season. Jerry Rice tried to come back after 15 weeks and promptly broke his knee cap (later admitting it was a big mistake coming back so soon). Then of course there was RGIII with a partially torn ACL, Washington put him out there, and against Seattle his knee was completely trashed and his career over after a sensational rookie year. But most guys aren't the same until a year after the surgery, or longer, even if they don't get hurt again.
My gut tells me Mahomes plays week 1 though. There were debates they rushed him into surgery (about 24 hours after the injury), and maybe into rehab (about 3 days later). He also didn't tear just his ACL, but LCL as well. Searches tell me there are debates on all this (of course). To me though the Chiefs seem to realize there isn't this huge window of numerous years left, and are sort of in a win-now mode. Witnessed by Kelce coming back for one more season, and KC paying him a whopping $12m. So if Mahomes can play at all week 1 and they don't think it will lead to another injury, they'll play him.