NPer( rate,pmt,pv, fv,type )

Returns the number of periods of an annuity with constant periodic payments and a fixed interest rate.

Parameters:

 Rate: Interest rate per period.
 Pmt: Payment received per period.  If you make payments, this is a
      negative number.  If you receive payments, it is a positive number.
 Pv: Present value of the annuity.  If you receive a loan, this is
      the loan amount as a positive number.  If you give a loan, this
      is the loan amount as a negative number.
 Fv: (Optional) Future value of the annuity at the end of the annuity.
      If you receive a loan, this is the final balloon payment as a
      negative number.  If the receive a final payment at the end, this
      is a positive number.
 Type: (Optional) Indicates whether payments are at the beginning of the
       period.
       True  = Payments due at beginning of period, with first payment 
               due immediately.
       False = Payments due at end of period. (default)