How long should I knead bread dough by hand?
Time isn't the main factor, gluten development is. You should knead the dough until the gluten is sufficiently developed to give the dough elasticity, and the final bread good structure.

There are two simple tests you can perform to check whether your dough is kneaded sufficiently. First, the Poke Test. Form the dough into a ball, and with a floured finger, give it a light poke. If it springs back to its original shape, you're on the right track.

Secondly, you can do the Windowpane Test. Grab off a chunk of dough, then try and stretch it apart. If the gluten is sufficiently developed, the dough shouldn't tear - it should stretch to a point where you can see light through it, hence the name.