Lots of people ask legendary investor Howard Marks the same question these days: How much longer will the bull market last? And they’re hoping that his answer will help them decide when to head for the exits. But he says that’s not a question anyone can answer with enough certainty. Instead, he says, investors are far better off making a different, more nuanced determination—what are the odds that stocks will continue to rise in the coming years?

If the odds are heavily in your favor, set your stock holdings at or near the high end of your long-term overall ­asset-allocation plan. If not, set your stock holdings closer to the lower end, but don’t abandon the market altogether.

To figure out the odds at any given time, Marks focuses on three questions:

What are the prospects for strong market returns? His answer today: Slim. The Standard & Poor’s 500 stock index has roughly quadrupled, including dividends, from its low in 2009. The easy money in this cycle has been made. Stocks have much less chance of producing such robust returns in the coming years.

What do valuations look like? His answer today: Not sky-high but still above average. This is not a market that is offering bargains.

What is investor psychology right now? His answer today: Stock investor behavior now is being driven by “FOMO”—fear of missing out. That’s why so many investors are piling into pricey tech stocks. How healthy can it be when investors readily admit an asset is expensive but invest anyway because it might go up some more?

According to Marks, based on the answers to these three questions, it’s clear that investors should be adopting a more cautious stance with stocks now.