I had miscarriage and d&c almost 6weeks ago. No periods yet. How long should I wait more? When shoukd I see doc abt it?

See your doc. You need to be evaluated by your physician. Everything is probably fine but you need a pregnancy test to confirm that your lack of a period is not because you have retained placental tissue in your uterus. If your pregnancy test is negative then your lack of a period is most likely because you have not resumed ovulation yet. Please use contraception until this issue has been resolved.
See a doctor soon. It is not uncommon for it to take 6 to 8 weeks for menstrual cycles to return after a miscarriage. This presumes normal / regular cycles prior to the pregnancy. Presuming no other problems or symptoms (no pain, no fever, no discharge) and assuming there are no pressing issues related to fertility, it would be reasonable to set up a follow up appointment with your gynecologist in the next few weeks.