Mr. van der Linden has just used the funds in his RRSP to purchase a 25-year annuity earning 8% compounded semiannually and paying $3509 at the beginning of each month. Mr. van der Linden made his last regular semiannual contribution of $2500 to his RRSP six months before purchasing the annuity. How long did he contribute to the RRSP if it earned 8% compounded annually?