211 - 350 for me (EDIT: 90 min days)
We regularly took A15/A16 past day 200. You know why? We had an end game. We used to stick with our map till we had assembled level 600 of everything for the entire group (4 of us back then, but only 2 remain). All guns, all armour parts. Level 600. That took us past day 200 quite often as it was quite hard to achieve (level 600 Magnums were a ■■■■■). Also A15/A16 was WORTH going past day 150 because only after that day did the game actually become challenging and the horde got amazingly huge. The A17/A18 hordes don't come close, and we rarely take A17/A18 even to a measly day 100 because we typically have everything by day 30, so there's no reason to go on except to push gamestage. And you can only do that so many times.
In A17 they drastically killed the game's longevity. This was because they decided that with enough perks the player can craft everything.