This is just based on my husband's experience. He was a very fit rider when he had 2 80% blockages fixed with stents. Not overweight, ate great. The cardiologist wanted him to keep his HR under 130 for about 6 weeks, which he sort of did, only because his HR is fairly low to begin with. He started walking 3 days after the procedure and was riding within 5 days; 2-3 flat rides and then slowly back to hills. We did a tour in Europe 5 weeks after the surgery.
If you feel fine and the doctor has not given you any restrictions, then just start building up mileage slowly, as anyone else would. I would suggest getting a HR monitor so you can judge how hard you are actually working.
Doctors always err on the side of caution. Set a goal and a reasonable timetable to achieve it. You don't need to be fast rider to ride a charity ride!