I've Googled this in the past for someone on here. Missing periods can be common and happens for many reasons aside from pregnancy. Google says if you miss 3 periods and are getting negative pregnancy tests, call your doctor.

over a week

My periods are always +/- 7 days. So unless I'm that late I don't worry

I dont get concerned at all until I "miss" two periods. If it gets to be a third in a row, I call my doctor. Many things impact cycle length outside of pregnancy.

One day is nothing to get excited about. Wait a week then take a test

I never started to wonder or worry until I was 5 days late. Bodies and cycles change for no explained reason sometimes