A good rule of thumb used in personal finance is the 4% rule. This means it is fair to assume you could make 4% off your portfolio a year consistently in income. While this is just a rough estimate, it is a great tool. This means for whatever amount of yearly income you want, multiply that number by 25, and that is how much you need to be invested.
So in this example, $500 a month is $6000 a year.
Multiply $6000 x 25 to get $150 000.
This means to get $500 a month from your portfolio, you would need to invest $150 000. This is very realistic if you invest consistently over a few years.