Just over 6 weeks for me. I have found out since that it was probably because I was doing too much - have you been "being really good" and running around doing stuff? Don't be shy about resting with your feet up and doing only just enough to look after you and the baby. Stuff the housework!
I have read that it is considered normal to bleed for anything from 1 to 6 weeks. I think a lot depends on the original position of the placenta and where exactly it was attached. See your midwife or GP if you are worried. I only bled for 1 week and only the first 2 days were heavy enough to need a maternity towel for absorbency - after that it seemed to be just a very light deposit of brown lochia stuff, so I tried normal towels, but quickly realised that the reason maternity towels are so thick is for the comfortable padding more than anything else!
I wondered the same thing too, I hadn't discovered mumsnet yet and thought it would all be fine - and then haemorrhaged at 6 weeks (retained products) kept bleeding heavily until about 12 weeks. I am really used to ignoring things and hoping they'll sort themselves out I wish I'd just chatted to my GP or a midwife earlier.