This question already has an answer here:
If bitcoin is ever going to be mainstream, how can we get around the problem of confirmation time? I don't think I like the idea of having to wait an hour for my transaction to be confirmed so I can leave the gas station after filling up. Even with the usual recommendation of waiting for only one confirmation, that's still 10 minutes. Anything over 10 or 20 seconds becomes a problem if all customers are using bitcoin, because it delays every transaction that long.