In short, even a relativelysmall dollop of stocks seems to be enough to provide decent assurance against running through one's savings too soon, while ratcheting up one's stock exposure doesn't appear to add a whole lot of additional protection against outliving your nestegg.
I recently had all my eggs in one basket and haven't wanted to buy things when they're relatively pricy, so I'm currently lagging in the small - caps, bonds, and emerging stocks.