14 days, 2 hours of play - but that is not a be all, end all.
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How To Request A Refund
Valve will, upon request via help.steampowered.com, issue a refund for any title that is requested within 14 days of purchase and has been played for less than 2 hours. Even if you fall outside of the refund rules we've described, you can submit a request and we'll take a look at it.
Additional information and troubleshooting instructions for existing refund requests can be found in our Common Refund Questions FAQ.
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If your machine can't pull a game, they are more likely to give a refund outside the window.
Also, part of Aces High is free - so I suspect the 2 hours won't start until Steam collects the coin. But then, who knows. Maybe not.
Regards,