That depends on a few factors. If you're looking for dividend income, you'd need to have enough capital to invest in dividend stocks with a yield high enough to generate that much income.For example, as of Nov. 27, 2023, the S&P 500 generated a 1.62% yield (meaning $1.62 per year in dividends) for every $100 you invested today. So, it would take $74,064 invested in an S&P 500 index fund at recent prices to generate $100 per month in dividend income. Long term, you'd also see your investment gain in value, and the companies increase their dividends, growing your income payout.You can also invest in a group of higher-yield dividend stocks, such as those held in the Vanguard Real Estate ETF (NYSEMKT:VNQ). With a yield of 4.5% at recent prices, a $26,664 investment would yield $100 in annual income and the same potential for capital and dividend growth over the long term.Savings and money market funds are back on the menu for yield now. With many online banks offering yields of 4% and many money markets yielding over 5% recently, between $24,000 and $30,000 in savings or a money market would yield $100 per month at recent interest rates.And while these amounts are safer in the near term and less volatile, interest rates -- which affect the future yield you earn -- may not be going up and could actually fall in coming years. You also don't get any capital appreciation from cash-based holdings, while the stocks of good companies become more valuable over time.