Why 30 and not five
Because a Bitcoin block takes about ten minutes, and a busy exchange can sit on a withdrawal for several more. A five-minute window would expire on half the payments made from an exchange account, which is where a lot of them come from.
And not two hours either: a window we cannot hedge is a window we would have to price into every item. 30 minutes is the longest we can hold at zero spread, so that is the number.