The long-running bull market could be on its last legs, says PNC's Bill Stone.

"I want people to get a little careful because we've gone so much longer than normal without even a 5 percent pullback," Stone said Tuesday on CNBC's "Futures Now."

According to the global chief investment strategist at PNC Asset Management, the market as of Tuesday had spent 281 trading days without a 5 percent pullback and 375 trading days without a 10 percent correction. That is significantly longer than the average 50-day streak for a 5 percent pullback and 167-day run for a 10 percent drop. The length of time without these dips has Stone expecting a reversal at some point in the near future.

"At the end of the day, you just want to get yourself positioned that you know that we're probably a bit on some borrowed time in terms of a sell-off," he said.

The strategist believes that two key factors could eventually turn the market rally on its head. First, Stone said, a "whole lot of policy [isn't] priced in now at all." While certain policies, like tax reform, have become a source of discussion for Wall Street, others like repatriation haven't actually made a splash in market outlooks just yet, he said. Furthermore, while global economic growth has been in a tailwind, if that were to reverse, then Stone would be "more concerned."

However, Stone stresses that in the meantime, U.S. economic growth looks strong and the global economy is also in a positive place.

As of Tuesday's close, however, only the Dow was higher for the month, though the S&P 500 and the Nasdaq were still hovering near all-time highs.