The gross number of taxpayers is irrelevant, the amount of tax revenues is the number we are concerned with. That number is $2.6 trillion.
http://www.taxpolicycenter.org/taxfacts/displayafact.cfm?Docid=200
Your number of "1.5 Trillion added per year" happened in one year only, fiscal year 2010. Previous years increases under Bush were more along the line of 500-600 billion. So your base rate is flawed.
http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt_histo5.htm
Start with the U.S. public debt at $14.5 Trillion and make some assumptions.
Let's cut spending so that our budget is more in line with what it was 10 years ago, or about 2.1 trillion dollars. Since tax receipts are 2.6 trillion for this year let's take that 'leftover' 500 billion dollars and pay down the principal on the debt. Assume the interest rate on the debt an average of 3.5% (which is much higher than it's been in recent history.)
http://www.treasurydirect.gov/govt/rates/pd/avg/avg.htm
Also assume the GDP is growing at 2.5% (much lower than recent historical rates in the 90's or 00's.) Let's assume that tax revenues would grow by this same 2.5% a year. This is actually a very low estimate if you look at CBO estimates here:
http://www.taxpolicycenter.org/taxfacts/displayafact.cfm?Docid=200
These are very simplistic calculations, very conservative in nature, but plug all these numbers into a spreadsheet and you have the debt zeroed out by 2044.
I've done a spreadsheet like this but I don't know how to post it to BB.