Hi
I had a double bypass 12 months ago, and my experiences may help (59 yrs).
I have been a keen runner/cyclist for many years, so the shock of being told that I had a blocked artery was enormous ("But it can't happen to me with all the exercise!").
I actually looked forward to the surgery and prepared carefully for afterwards.
A bike was set up on the turbo trainer in the garage as I knew that I would be forbidden to cycle outdoors for a while. On the day of surgery I rode a gentle 30 miler "to keep my lungs clear".
After a total of 4 days in hospital I got back on the bike next day and did a very gentle 2 miles - this was mainly to make sure that getting on and off was OK. Within a week of returning home I was cycling 10 miles on the turbo and walking 4 miles.
The surgeon allowed me to cycle outdoors after 4 weeks - you can't believe how liberating this felt after weeks in the garage!
To shorten a very long story I have cycled every day since leaving hospital, usually about 150 miles per week, and always including a ride of 50 miles+.
Recently, I returned to gentle running. This is very slow (11/12 mins pace) but is much harder work than cycling where you can freewheel regularly to recover. I am now running 2 x week about 3/4 miles each time.
So, in a few words my answer is AS SOON AS POSSIBLE!
I would be happy to provide more detailed information if you want to get in touch.
Rgds
Pete
p.lucas at burghley.net