RT window you can keep 2 min. That is 18 min-22min. About means 10%

Thanks...but is there any reference for this??

It means the retention time by HPLC or GC was about 20 minutes (Probably HPLC, but you did not specify the analytical technique used).

For HPLC

About does not mean 10%???? Sometimes poorly written documents are unclear (not much you can do about that now), but inferring a value onto something with no stated value servers no purpose.

Medhat, perhaps you could tell us what your real question is since the post is not clear.

As for compendial method retention time given in the method is only for the information. It may vary for different column manufactures of same USP category. You should report in your methodology what is obtained by you and validate the method.