So you could take a table in excel - here's what $10,000 a year does at 7% - and then look at where you feel like it's substantial. For an arbitrary cutoff, 12 years is the point in which 7% returns can be expected to compound on themselves faster than your original contribution increases the balance. But it's really kinda continuous.
As you continue to be a badass in your life your expenses will effortlessly drop, too, so the same amount of money goes further. From MMM's example I'd say that can be expected to continue after retirement.