There are about half a million minutes in a year, so 50 million seconds is a year and two thirds. At the rate of saving 50 million seconds a day, in a year you'll have saved around 608 years—which is only a dozen lifetimes if a lifetime is around 50 years. Still, that's a pretty close approximation for an off-the-cuff guess.
I'm sure he'd have planned or thought about this before hand.
Steve's famous "computers are a bicycle for the mind" was refined over a long period of time and countless interviews. We only hear about the one time where he perfected it, where it made an impression. Many other instances are on YouTube, in one you can see him trying out different alternative lines.
The problem is that while it's an impressively close approximation for an off-the-cuff guess (at least if we charitably translate his "dozens"/"dozen" to 12), to the extent it was pre-planned it's a terrible approximation. ~50 years as a lifetime is the right order of magnitude (and thus a very good result for a guess), but is too far off to be any good if precalculated.
