^^The valve site says - " The friend can then play the game for a limited time. Once the guest pass expires, they will be prompted to purchase the game in order to continue playing. A guest pass is an easy way to let your friends try out your favorite games with no commitment or expense on their part."
and the expiry date is shown 22-9.So it appears to me that you can play only up to then.I don't know much more as this is the first pass I have got.