general rules of pulling wire: grab it in one hand and stretch it across your chest until your other hand is fully outstretched (like your body is a "T"). Unless you are really tall, this is usually around the 5 foot mark, probably slightly more. Pull out as many outstretched arm lengths as you need in increments of 5.
The extra length helps with possible tears of the jacket when pulling it through the wall/conduit by taping a drag string to the folded over end. Cat6 jacket is flimsy and tears pretty easy. When you go to land the connectors, it's also nice to have an extra foot hanging out of the wall that you can easily stuff back in afterwards, too.
When you're done pulling a section, mark the box with the remaining length of wire left in the upper left corner of the box. Use the poke hole in the upper right corner to stuff the pulling end so it doesn't get lost back in the box.
Note: If pulling multiple lengths at a time, number both ends of each before pulling through the wall so you don't have to test afterwards.