Bitcoin transactions over Ripple Network
As you already have described, Ripple transactions are based on exchanging IOUs. As such, when you trade bitcoins on the ripple network, you are trading IOUs denominated in Bitcoin owed to you by some party, e.g. BTC owed to you by Bitstamp. Therefore, as long as the transaction is between Ripple users, it is completely off-chain and can be confirmed within seconds.

However, there is at least one Ripple Bitcoin bridge which is offered by Bitstamp. This bridge allows Ripple transactions to directly end in on-chain Bitcoin payouts. It works by having a Ripple transaction send money to Bitstamp, who then converts the amount and sends Bitcoin to the recipient. The Ripple side takes the regular short time to confirm, the Bitcoin side is a regular on-chain transaction though, which will be confirmed as usual in the next Bitcoin block.