After a heart attack and stent, how long before someone should be able to part in strenuous activity?

Depends: cardiac rehabilitation is used in those with coronary artery disease. Your question is dependent on a number of factors and there isn't a simple answer. People can have what's called silent ischemia depending on risk factors thus it is tough to tell withy certainty what is considered too strenuous for ones health. I would discuss this with your cardiologist. Define strenuous with them.