I've yet to stop growing older, ergo I've yet to stop training. My training spans five decades now. I was told that if I could keep training on a regular basis for 6 months, I could keep training for my entire life and so I have.
Outside of some kinda serious accidents, such as being hit by a 20,000+ pound crane and dragged by it for about 30 or so feet

or the rare vacation, I rarely miss training. I now train between two and five times a week for about 30 or so minutes. Primarily H.I.T. or a variation thereof. How do I know when I've missed "enough" training?
I miss it.

If you stop forever, I can pretty much guarantee you will forever regret it. I know this to be true as I occasionally run into former gym mates that no longer train. They are reeeeeally fat, or skinny-fat and on top of that they feel awful. They tell me they wish they had never stopped and while some have good reason for having stopped (severe injuries not training related), most just walked away from it because they thought they no longer had anything to gain.
I tell them that as we age it's not so much about what we have to gain but rather what we have to lose. Training slows down the loses, if you will.
Exercise because it is a part of your life that adds to your life. It isn't all of your life and never should be. There are plenty of smart folks here that can inspire you more than I and there are a few idiots than can still inspire you more than I can.

Like I said, it should add to your life, not take away from it.