Minimum 5 years...
after the 2019/2020 season we will only have Schröder, Bembry, Prince, and Collins under contract. I bet we don't sign any free agents that will be on the roster after that season. We have 6 1st round draft picks over the next 3 years that we will either use or package to move up plus 2nd round picks. The Hawks will hope that they have at least 3 starters and some bench players from those picks and our current young guys, then sign a couple good free agents to round the team out. It will take a season to play together and figure out if we need another piece or 2. 5 years to field a playoff team. Plus that allows the current stud teams to get past their prime.
I'd also be surprised if Dennis is still on the team. If we won't compete for another 5 years we might as well trade him.