I'll take a stab at it. $100K is low end substantial and $3.2M is high end substantial. The administration is proposing doing away with RMD for accounts under $100K and capping accounts at $3.2M.
They also have a proposal to require RMD for Roth accounts..
I would go a little higher and say that about $250k is the beginning of substantial. There is no higher limit on substantial, but due to the limits of 401(k) contributions an individual with $3+ M is either very highly paid, or a very good portfolio manager.
While there is no towering giant in the field of portfolio management in distribution, like there are in the portfolio management in accumulation field, the very best advice I have seen is that ~5% of the current value of a portfolio can be withdrawn every year without too great a risk of depletion. I am hoping to live with 4% annual withdrawals, which would be much more safe. But then I am a conservative and that is what we do. But, the portfolio must be almost entirely in stocks for that to work. The 40% fixed income - 60% stocks I see from most advisors does not allow that kind of withdrawal.
Even a meager income from a pension or SS allows variable portfolio withdrawals to be a very viable strategy in distribution.