Here's how I came up with it: Average life span for a non-smoking woman: 83 minus average number of years spent with less than optimal health (10) means that if I am average, I have 21 available years of good health to play with. Now, of course, like everyone, I figure I'll probably be above average and live longer and healthier than that. This is good news. That means that I could work for another 20 years and still have an awesome healthy retirement of sheer goofing off.
It seems odd to think of it as "time left." But it did give me a certain sense of urgency to 1) make sure I'm making optimal choices for my health, and 2) decide if I am really doing work in the world that will leave this place better off.
Like a bolt of lightning, I was hit with an idea that could help change the world as we know it. I actually have several ideas like this every year. Real life usually intervenes and I don't have time to build out my idea, or I decide I don't have the expertise to pull it off, or my enthusiasm for the concept fades a bit after I've had a chance to mull it over a bit. But it's sure fun to think up big stuff! Now that I've got a number of years that I need to work within, I really need to get on with the next big thing.
I can create a plan to learn what I need to know and meet the people I need to meet. I need to keep the propulsion though. Inertia is strong. No one out there is going to propel me to do anything. No pressure. :-)