How long after a Breast Augmentation can I get back to playing tennis / lifting weights?

Your plastic surgeon will always be your best resource when it comes to specific instructions in regards to returning to activities. Generally speaking, best  to limit strenuous activity for at least 2 weeks after breast  augmentation surgery, assuming you are doing well and there have been no complications.  
Care should be taken during the first two weeks after surgery not to allow for elevation of heart rate, blood pressure, and/or direct trauma/contact to the chest wall. Generally speaking, most my patients are able to return to lower body exercise at the 2 week post operative mark, wait to lift/run for about 4 to 6 weeks after surgery.  Other exercises, involving the pectoralis major muscles, may resume several months down the line, if not longer. In my practice, I would ask that you avoid activity such as tennis or golf  for at least two months after surgery.
In other words, start slow and "listen to your body" as you gradually progress in intensity/duration of all activities. Best wishes.

This will vary between surgeons, but I ask my patients to hold off on upper body workouts for 4 weeks after surgery.  I then recommend you start slow with light weights and gradually work up to what you were at before.  If it hurts, don't do it.  Just listen to your body.  You can surgery and you need to allow adequate time for your body to recover.  Of course you should always follow your own surgeon's instructions post op.  Best of luck!

Every surgeon has a different recovery protocol but I usually have patients return to full activity within 4-6 weeks after surgery.  By the time sumner arrives you should be in good shape to play tennis