Not very high. It would destabilize its price and wreck the supply model Bitcoin adheres to, which is curently unnaceptable to most nodes. Not to mention the fact that PoS alone just doesn't work, it has to have either central (Peercoin) or elected (BitShares) authorities to help produce blocks, or be combined with PoW, for instance. But that's OK - we have newer cryptocurrencies to solve these kinds of problems. They won't stop Bitcoin from remaining the absolute digital gold for some time though.