Bogle has gone on record [1] that he's comfortable as long as indexing doesn't account for 75%+ of the market (I suspect he means 75%+ of market-wide AUM, but interpretation of "the market" can vary between people, like total daily trading volume or total daily trading buys, YMMV).

[1] http://www.marketwatch.com/story/john-bogle-has-a-warning-fo...