At least 30 minutes in a day, probably 3 days a week at minumum as the body requires it to maintain itself that is though if you want to live a happy and good life, but then over doing its isnt good as well, theres need to be a balance in everything
For me that's 20 minutes a day on a rowing machine plus some body weight strength exercises.
Which is about a YouTube video long and as a result I basically only watch YouTube content when I'm doing it (hey I'd like to watch this -> I should go start rowing) is a surprisingly good motivator.
For example if you are doing arm exercises Monday, don't do arm exercises again Tuesday, work another muscle groups like your legs.
I had to learn this the hard way when I was starting out..
https://doi.org/10.1016/j.jshs.2020.04.001
As for heart rate, going above threshold is never comfortable. But it's a good pain and you can get used to tolerating it for occasional high-intensity interval workouts. Over time this will decrease your resting heart rate and reduce the risk of a major adverse cardiovascular event (MACE).
Recent research highlights a number of things, but what sticks out is that it's important to maintaining muscle mass because we lose a bunch as we get older, and loss of muscle is a leading morbidity factor.