If someone was looking to strengthen heart muscle, what parameters they should target and for how long.
I know you have do exercises which will raise your heart rate but not sure about the intensity or the duration.
You need to strengthen your entire body. You cannot just strengthen your heart muscle. No doubt you already know that.
Who is it for? Why? Rehab after heart condition? Lots of questions. How strong/fit is the person currently. They may need to start off very gently over a few weeks before ramping up intensity.
They may have some other physical weaknesses that must be addressed prior to building up the exercise workload. Engage the services of a good - and I mean good personal trainer. Check them out. Discuss your needs with them. They will be able to best assess your exercise requirements as what you are asking is far too dangerous to be discussed over a forum.
In addition, include your doctor (if you are currently under medical supervision & drugs) in your (or other person's) endeavours as they may need to lower certain medication as you build your strength & fitness.
Hope that this helps you to understand the very basics in simple terms.
Just uses common sense and you will achieve your goal(s)