It is likely to be sooner than something that has to start from scratch. Of course you don't have to start from scratch, you could build something similar on top of the BTC block chain. But even then, since the client software is open source already, you could use it and modify it make it talk to a block chain based back end.

And even if the Ripple server were never to be made open source, it can still be used *right now* to help Bitcoin grow. After all, for now you also have to use traditional payment systems to get BTC.