The S&P has just marked its latest milestone by crossing the 2,000 level. So how long will it take for the benchmark to add another 1,000 points? Actually, according to one well-known strategist, S&P 3,000 could come in just five years.

Morgan Stanley strategist Adam Parker is predicting that “this could be the longest U.S. expansion ever” and predicts that the S&P will rise to 3,000 by 2020. Parker cites low volatility and interest rates, better corporate management, and sustainable growth rates as among the evidence that “suggests the US cycle peak is far from imminent”.

(Watch: Morgan Stanley bull sees a clear path toward S&P 3,000)

But how does Parker get to 3,000? It’s actually rather simple. The strategist’s call is based on an assumption of 6 percent annual earnings per share growth with a 17x price-to-earnings ratio.

Still, not everyone is convinced.

“I don’t think it’s possible” for the S&P to rise 8 percent per year, said Gina Sanchez, founder of Chantico Global.

Though Sanchez sees some growth in the S&P 500, she doesn’t think it will hit 3,000 so quickly.

“Assume for right now that we’re running at a trend growth rate in the U.S. of somewhere around between 2.5 and 2.8 percent,” said Sanchez, a CNBC contributor. “Add to that maybe 2.2 percent of inflation.”

Even using the higher of the growth rates in Sanchez’s estimates yields a post-inflation nominal rate of just over 5 percent. She then adds two more percentage points for dividends to get a 7 percent figure.

(Watch: US stocks finish mixed; S&P 500 clings to 2,000)

“That assumes you’re at the right valuation,” said Sanchez. “We actually see some contraction over that period. The numbers that I tend to work with are something around 6.2 percent as base case. That doesn’t get you to 3,000 until about 2022. That’s two full years after Morgan Stanley, assuming that we’re going to get there.”

For Parker’s growth rate in the S&P 500 to transpire, GDP will have to be above 3.5 percent and inflation will have to come in at least at 2.5 percent, Sanchez said. “I don’t see that happening. I don’t see how you can justify that.”

Craig Johnson, senior technical analyst at Piper Jaffray, also believes Parker’s projection for 3,000 by 2020 is aggressive, though he believes that level is within reach.

“The big base we broke out of is setting us up for the secular bull market [which] can move higher based upon history,” said Johnson, who notes that S&P 500 nearly quintupled from the early 1950s to the mid-1960s, and from 1982 to 1999, rose to twelve times its price in just 17 years. “The target will be achieved. The question, really, is time.”

To see the full discussion on the S&P 500, with Sanchez on the fundamentals and Johnson on the technicals, watch the above video.