You can get this achievment in chapter 4, after you help Bruno you can talk to some npc's in the market and do some mini games, one of them you need to collect 20 materia around the market. You need to collect them all within 14 seconds to get this achievment.
Comments from TA:
1) It seems that you need to grab them within 14 seconds, not within 15 seconds. The timing on this is really tight.
You'll know you've accomplished this in-game when you are rewarded the full amount of 100 Gil and Aerith remarks something like "Wow, I didn't expect you back so quickly!"
I have no idea why Square insists on putting minigames like this one into their RPGs when the gameplay controls and camera aren't well catered to the minigame objective; this minigame reminded me of the dreadfully-controlled Calm Lands minigames of Final Fantasy 10.
2) Can also do this in chapter 7 when getting parts for the flower wagon if you missed it in chapter 4