That's the major milestone he has to meet, but I don't know if it was clear whether he could hit 50% done 1 day prior to the 5 year mark and have made the notice in the 5 year window, in which case he might have 6.5 years in total.
We don't know the whole set of milestones he has to meet though. If he doesn't win the auction, you have to think he's either already out or has to, very quickly, find a new location and secure it with all the approvals for building.
I'm curious if his 5 year window just goes away as soon as he misses something or if he gets to retain the intellectual property rights to the team for the remainder of the 5 years and then it reverts back to the NHL. Importantly, it could mean he's out and loses it, so another group could step in sooner.