One way, as Jim pointed out, is to build an amortization table.
If you are simply increasing the payments, you can use NPER to determine the
revised number of periods.
If the payments are irregular (and especially if there's just a few), you
still could use financial functions to calculate your result. Whenever you
are making an irregular payment, treat it as a new loan. Use FV to calculate
the current balance on the loan, then NPER to calculate the number of
periods remaining after the additional payment.
But if you want to do a what if analysis, I'm with Jim, an amortization
table is the best way.
