Period Length (help!)

When it comes to my period... I only actually bleed for the first 2 days. For the next 5-6 days after that (if there's ANYTHING at all) all there will be is super light in heaviness, light brown discharge. My question is: what is my actual period length? The time there is blood? Or the entire duration of the the brown stuff too? Like seriously, there's hardly any of the discharge, but it stops about one week after I began bleeding. All period trackers take your period length into consideration when it comes to your fertile days. And I don't want to get it all wrong because I didn't know how long my actual period is.