It doesn't take any time. I open a card, pay my rent with the card to get the bonus, pay off the card (no interest), and then put it away and cancel a year later.

Edit: Also, I consider it a hobby. It's fun to try to beat the system. And the reward for "beating it" is incredible experiences (i.e. Singapore Suites) that would be far out of reach if I didn't play. Would you ask the same question of someone who cooks or plays video games what their ROI is?