I'd suggest not spending so much time that you'd have to basically hit the lottery to not lose money on it. There is no "correct" amount of time, but the odds of losing money is really really high when you spend 14 months on an iPhone game. They happen to hit that lottery and so they are doing well, but the odds were not in their favor.

Take a look at your expected payout and budget your time and money accordingly. I know the app store is like a lottery, but you can make it so that your default is closer to break even with a lottery ticket, instead of default of losing tons of money with a possible lottery ticket.