So maybe it would be smart to open accounts in several exchanges, just to be ready. Best would be BTC to Paypal. Or is there some "clearance time" necessary to process the transaction?
You can buy and sell bitcoins for real money on exchange without waiting if you have the required funds. That means that you made a bank deposit if you want to buy them (could take up to a week to proceed), or if you deposited your bitcoins.
But then, when you sold your bitcoins in the exchange website, it could take several days before you will get the money in your bank account.
It does not really matter, because even if the price reach 0, you still finished your transaction.