before my first child at 28, I had 5 days long with painful cramping AFs. After having her, it was still about 5 days, but no more cramping at all. After my son at 31, they went down to 4 days and a bit lighter. Since my m/c at 38, it changed dramatically and Af is now about 2 days with one day normal/heavy flow, one light, and sometimes some spotting on day 3, sometimes not.
It did worry me, but my consultant didn't consider it a problem and I've read in many places that it is ok to have a 2 day AF, so now am just grateful for it!