I think it’s a useful reference but not a magic number. You could continue to work and explore interests, hence getting constant stream of active income. You could also need large expenses.
$Tesla Motors(TSLA)$ bulls have previously backed the company and Elon Musk by saying how it is a tech company not simply a car company. However, after years of overpromising and under delivering, retail investors should now see Tesla for what it truly is--an EV car company. Its true valuation is probably way lower. I think it's a good stock to trade, not to invest in.