The market is currently slightly over-valued now which is reasonable since stocks offer a much more attractive return than bonds due to low interest rates. Eventually, however, interest rates will get to levels of at least 4% (which is the minimum normal rate on interest rates) and that would justify a P/E closer to 15. I am no prophet but if I had to guess, I would think future returns will be somewhere between Bogle's and Shiller’s estimates, i.e. between 8% and 10%.