Celtics.com has a nice article on Tony Allen's rehab. Says he's on pace to come back by training camp (but somehow that seems optimistic to me).
ACL injuries typically require about 12-18 months of recovery time, but everyone's body is different, and different knees can be pushed harder and respond differently during a rehabilitation process. With swelling that completely hid his knee cap, Allen started his rehab right after his January 13 surgery with light, non-weight-bearing exercises. The workouts slowly picked up in intensity as he regained range of motion in the joint and strength in the surrounding muscles.
Allen is expected to be ready to go in time for training camp in October.
I just hope he takes his time and gets it right.