I think a minimum of 6 months followed by a successful Exercise Tolerance Test.
Also known as a stress ECG. Supervised by a Cardiologist with a written report & traces sent to Gatwick Medical Department.
You will need to get down to a gym and get some practice on the treadmill, once cleared to do so by the medics. Swimming helps. Need to exercise 5 times a week. Get your diet & weight right as well.
Look up the BRUCE protocol as to the speeds/slope. Become comfortable with the build up stopping after 12 minutes.
Over 12 minutes requires you to run which I would personally avoid !
But Speak to your AME in the first instance.