“How long after a heart attack can you exercise?”

Generally 2-3 weeks after MI you can begin graduated exercise ( start slow)
Consider cardiac rehab to guide you

I guess it depends how you define exercise. In general, after a heart attack my recommendation would be that you should be active including walking possibly bike riding, and other activities like golf etc. I would recommend keeping your activities such that the heart rate does not exceed 120 bpm for the first three months. After three months you can begin to accelerate that and eventually at 6 to 12 months resume full exercise activities other than heavy weightlifting.

Depends on the case but usually about a week to ten days you can start ! Check with your cardiologist!