Short answer: As muych as you can.
According to a British-led study published this week in PLOS Medicine, if you want to maximize your heart health, there is no threshold for how much physical exercise you should be doing.
In short, argues this study, the lowest risk of cardiovascular disease is seen in those people who are most active.
Bottom line, though: Any activity is good.
More activity is better.
But only if you can and if you like doing it.