Are there any real statistics on how many people fall away? I recently came across the claim that 80-90% of people who convert to Christianity will eventually revert to their former beliefs/way of life. While I can't speak to the world at large, my experience is that this number is closer to 40%. I was wondering if there were some actual numbers one could look up to confirm this. I was also wondering if there were a way to somehow subtract the number of people marrying into the faith (as that accounts for a number of baptisms/confirmations too).
Short answer: 

It would appear that the answer to your question that is reliable, and applicable across the board, is a resounding "no". Within denominational or individual Church/affiliation boundaries, there are plenty, but the numbers vary



Explanation of the short answer:

I've found several sources that give "fall away rates" with varying degrees of source citations.
From http://www.frontlinemin.org/decisionism.asp

(and I'm not sure if these numbers are trustworthy - a number of them are vague and dont' give the Church, Crusade, denomination, at all.  They could easily be fabricated.)


  
  Charles E. Hackett, the division of home missions national director for the Assemblies of God in the U.S. said, “A soul at the altar does
  not generate much excitement in some circles because we realize
  approximately ninety-five out of every hundred will not become
  integrated into the church. In fact, most of them will not return for
  a second visit.”
  In his book Today’s Evangelism, Ernest C. Reisinger said of one outreach event, “It lasted eight days, and there were sixty-eight
  supposed conversions.” A month later, not one of the “converts” could
  be found.
  In 1991, organizers of a Salt Lake City concert encouraged follow-up. They said, “Less then 5 percent of those who respond to an
  altar call during a public crusade . . . are living a Christian life
  one year later.” In other words, more than 95 percent proved to be
  false converts.
  A pastor in Boulder, Colorado, sent a team to Russia in 1991 and obtained 2,500 decisions. The next year, the team found only thirty
  continuing in their faith. That’s a retention rate of 1.2 percent.
  In November 1970, a number of churches combined for a convention in Fort Worth, Texas, and secured 30,000 decisions. Six months later, the
  follow-up committee could only find thirty continuing in their faith.
  A mass crusade reported 18,000 decisions—yet, according to Church Growth magazine, 94 percent failed to become incorporated into a local
  church.
  In Sacramento, California, a combined crusade yielded more than 2,000 commitments. One church followed up on fifty-two of those
  decisions and couldn’t find one true convert.
  A leading U.S. denomination reported that during 1995 they secured 384,057 decisions but retained only 22,983 in fellowship. They
  couldn’t account for 361,074 supposed conversions. That’s a 94 percent
  fall-away rate.
  In the March/April 1993 issue of American Horizon, the national director of home missions of a major U.S. denomination disclosed that
  in 1991, 11,500 churches had obtained 294,784 decisions for Christ.
  Unfortunately, they could find only 14,337 in fellowship. That means
  that despite the usual intense follow-up, they couldn’t account for
  approximately 280,000 of their “converts.”
  


But another source gives different statistics, way off from the ones above.

(and still, I wonder where they get their numbers.)

From http://biblicalinsights.net/Book%20Reviews/Falling%20Away.pdf


  In chapter one, Simmons establishes the reality of falling away.
  Reliable surveys have  been done which show an apostasy rate as high
  as 40% among adult American Christians.  Others show a rate as low as
  10-15%. The average apostasy rate is 25% (3). Catholics have a higher
  apostasy rate than Protestants. In the Church of Christ faith
  tradition, the apostasy rate is 50% for individuals 18 to 25 years old
  (3). It is estimated that over 75% of all Mormons will fall away from
  the faith at some point in their lives (3).


As for the anecdotal evidence supplied from the teaching you linked to in your question, Mr. Comfort gives the source for that particular statistic as a copy of American Horizons Magazine, the official magazine of the Church in question. (Coincidentally, the last one in my first bulleted list...)

From http://internet-churches.com/true-false.htm


  I was reading in the American Horizons magazine, which is the official
  magazine of a very large, or a major, denomination of the United
  States which has 11,500 churches throughout the US - in 1991, their
  first year of what they called “the decade of harvest,” they got
  294,000 decisions for Christ. They found that only 14,000 remained in
  fellowship. That is, they couldn’t account for 279,000 of their
  decisions for Jesus. And this is normal modern evangelical statistics
  when it comes to crusades and local churches.


But as for an official, scientific study, or a survey that follows real methods, I find no evidence.  It would appear that the numbers are based on anecdotal evidence, or as Mr. Comfort states. "Church records" - records shared from Pastors at Churches.   Even in the Hell's Best Kept Secret teaching, he states that the number is based on Church records that he has had access to. He never claims that this is a universally applicable statistic.

This is, at least, feasible.  I know that in our Church, we keep records of those who have prayed the sinner's prayer, and we have records of who is still in attendance, and who has left, and for what reason. (Church discipline, asked to be removed from membership, etc.)  I'm sure if I talked to my Pastor, I could come up with a calculation for our own Church's "fall away rate".  

But is that indicative of the fall-away rate for Christianity as a whole?  no.  

And given the fractured nature of Christianity, and even the difficulty in defining what "falling away" really means, I'd say it's not feasible to get such results. How would you define "falling away"?  No longer attending Church? What about those that don't attend a Church at all, but are active in prayer, evangelism, and just don't want to go to a local Church?  If not based on active Church membership, then what?