How to get execution time of a script?

I have a program to report the result of MRP. It need to process 8000 materials. I used loop to process them one by one. Now, I want to know the processing leadtime for each material. I placed following code

loop at out_put.

write: <b>sy-timlo</b>, out_put-matnr, /.

endloop.

But, unfortunately the timestamp in output is all same though the program ran for 10 minuties.

So, anyone can told me how can I get the actual execution time for each material. Thanks.