What is the best passive investment strategy?
Yes, I would save $500 per month and invest it on SPY. This is an ETF wich resembles the S&P 500 and has some of the lowest expenses. It is reasonable to assume a return of 7% per year over 30–40 years. With that, you will have over a million dollars.
Saving like some Europeans, or 20% of your disposable income.
Saving like some Asian, or 50% of your disposable income.
The data is here: Household accounts - Household savings - OECD Data
Imagine that by doing so you are able to save $12K per year. Then you will get over $2 Million. If you are able to save $24K per year, you will get over $4 million
By being a passive investor, on a well diversified portfolio (the S&P 500), you are more likely than most to do well