What's the formula to find duration/years to recoup an investment

The formula is complicated, but what it boils down to is: Duration = Present value of a bond's cash flows, weighted by length of time to receipt and divided by the bond's current market value. For example, let's calculate the duration of a three-year, $1,000 Company XYZ bond with a semiannual 10% coupon.

Estimate the sum in two different ways:

A) By rounding off to the highest place value (hundreds):

 368 = 400  (3 hundreds is the highest place value)

 231 = 200 (2 hundreds is the highest place value)

 

 400 + 200 = 600

B) By rounding off to the second highest place value (tens):

   368 = 370

   231 = 230

   370 + 230 = 600

Hope this helps!