The two rules on this are you have to have purchased the game less than 14 days prior and have played for two hours or less.
Also worth noting...
"Refunds are designed to remove the risk from purchasing titles on Steam—not as a way to get free games. If it appears to us that you are abusing refunds, we may stop offering them to you. We do not consider it abuse to request a refund on a title that was purchased just before a sale and then immediately rebuying that title for the sale price."
http://kotaku.com/steam-is-now-offering-refunds-1708492000
Also worth noting...
"Refunds are designed to remove the risk from purchasing titles on Steam—not as a way to get free games. If it appears to us that you are abusing refunds, we may stop offering them to you. We do not consider it abuse to request a refund on a title that was purchased just before a sale and then immediately rebuying that title for the sale price."
http://kotaku.com/steam-is-now-offering-refunds-1708492000