You may want to discuss this with your therapist. It's best to work out a schedule that will enable you to exercise throughout the day at regular intervals. It's understood that
early on in the rehab program patients are still recovering from the operation. This is also the crucial time for getting as much motion as possible.
Maintain a good diet and get plenty of sleep at night. Check with your doctor about your fatigue level if it seems excessive. It may be drug-induced or you could have some other medical reason for it.
Some patients choose two times during the day when they set aside what they are doing and carry out their exercises. Others prefer to set a timer and do one or two exercises every hour. Your therapist will be able to advise you on which is best for you depending on how much joint motion you have.
Find out what your joint motion is and how much you need to increase it for each day, week, and over the next month. Make that your goal and work towards it everyday.