It'll happen. The only thing more common than a 15-25% correction is a subsequent rebound.
So long as you view an all time high and the inevitable downturn as equally temporary, you'll be fine.
As to exactly when: if I knew that (which I most definitely do not), Warren Buffet and Bill Gates would already be calling me "Mr. Basket Case," or just "Sir," for short.
Seriously, if your time horizon is less than a typical business cycle (6-8 years is normal), you don't need to have more than 30% of your portfolio in stocks...unless you have an unusual level of highly reliable independent income.