ITOT vs CDs ... 15 years away from retirement

Anyone know, what is the longest duration when the market was down, investors just lost money.

I am 15 years away from retirement, and thinking should I be investing 50% in FDIC insured accounts like CDs and 50% in ITOT.

Just really nervous to invest more than 50% unless data shows there are roller coasters but eventually market recovers after a 'short' downturn.