Ad and Gem Location and Time Required

Immediately to the right after clearing Stage 1, a roulette appears. Time required to get here is less than 1 minute. You can spin up to 3 times by watching ads every few tens of seconds, with a 50% chance to get 1 gem. After that, it seems the spins revive after about 3 hours and 45 minutes. I'm not motivated for the stages, so I only plan to do this super easy quest.