Players on PUP can begin practicing after Week 6 of the regular season and then have a five-week window during which they can return to practice. Once they begin practicing, teams have three weeks to decide whether to activate them. ESPN's Mike Sando has the full explanation of the PUP rules.
UPDATE: Per the NFL’s official transaction wire, both Dorsey and Reed were waived/injured. That means the team will eventually work out injury settlements with both players, which would pay them for a portion of the regular season that they would have missed due to their injuries. While it’s possible the Packers could re-sign them, injury settlements often mean the end of a player’s career with that team.