"Bull markets are born on pessimism, grown on skepticism, mature on optimism and die on euphoria," said legendary investor Sir John Templeton. So, when will Wall Street know this nearly-10-year-old bull market is coming to its bitter end?

According to BofA/Merrill Lynch Global Research, one indicator suggests there's no cause for concern just yet. In December, the Sell Side Indicator, which is BofA's measure of Wall Street's bullishness on stocks, rose for the third straight month to 56.8 from 56.1. That puts it at the highest level in more than six years.

While market optimism would often be considered positive, markets that become so optimistic they verge into euphoria can be a death sentence for a bull run. At this rate, that could mean as little as one year left in this bull market.

"The rapid improvement in equity sentiment over that last 16 months has continued to push the indicator further into 'neutral' territory," analysts said in a Jan. 2 note. "And while we're still a long way off from the current 'sell' threshold of 62.8, at the current pace, we could reach those levels by January 2019."

The Sell Side Indicator is derived from the average recommended equity allocation of strategists on the last business day of each month, BofA said. The measure has historically been a "reliable contrary indicator," suggesting a bullish Wall Street could indicate a bearish market and vice versa.

When the indicator has been this low or lower, BofA said, returns over the next 12 months have been positive 93% of the time. Median 12-month returns in the historical period were 19%.

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