It depends. If you plan to build a profitable startup, meanings, you are willing to sell the service (for $$) as early as possible, then perhaps, "18 months" i

It depends. If you plan to build a profitable startup, meanings, you are willing to sell the service (for $$) as early as possible, then perhaps, "18 months" is a period of time which within you get to the break-even point. If you are planning to build a cool-and-free-iPhone/android-app and all you care is to get as many users as possible, then perhaps, this is a great advice for you. In the first case, however, it means, you only need raise enough for about 12-18 months.