A rotator cuff rehabilitation program should be implemented at different stages of a shoulder condition, depending on which one. For example if you suffer from a tear, you may start rehabilitation right from the start together with anti inflammatory drugs or ice packs.
Physical therapy improves the function of the 4 muscles of the rotator cuff. For this no heavy lifting is required at all, only specific isolation movements that target the cuff, not the deltoid above it.
Likewise, if you suffer from Impingement, Bursitis, Tendonitis or Adhesive Capsulitis (Frozen Shoulder) you may start rotator cuff rehabilitation right away. If you just had surgery, it is a different matter though. You cannot start right away, but you can achieve improvement in 4 different phases.
The first is the Passive Phase lasting up to 6 weeks, where the therapist moves your arm for you so as not to contract the muscles. This is because the tendons are still too weak after surgery to bear a load, so no tension in placed on them.
The second is the Active Phase lasting up to 12 weeks, where you, the patient, can start moving your arm actively, but without any resistance.
The third is the Strengthening Phase. This phase is very important because the cuff has become weak after surgery, and a set of specific exercises must be performed with only light resistance. No heavy lifting is required as in bodybuilding military presses, because they target the deltoid, not the rotator cuff.
The fourth is the Full Activity Phase lasting up to 6 months. In this phase full recovery and functionality is restored, though it may vary on individuals.