Filing for I-140 after PERM approval

Hi, I got my PERM approval on 1/10/2018. Priority Date is 8/9/2017. My 6th year of H1B expires on 11/8/2018. My HR is saying its recommended to file the I-140 only 6 months before the current H1B expiry. is that true? or is it that the H1B extension must be filed 6 months before current H1B expiry? Kindly clarify.