Is there a general rule to the amount of time I knead dough? Some recipes will say "until it is considered soft and smooth". Regardless of what the recipe claims, I am kneading the dough for a minimum of ten minutes when it asks for it.
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.
I agree with ElendilTheTall that we should not use a fixed time to identify a well-kneaded dough. A lot of factors such as brand and type of ingredients used apart from weather climate and temperature plays a part in the development of gluten. If you want to see how a well-kneaded bread dough should feel and look like, you can check out my link, where I have posted some pictures and videos to help. http://eelistan.hubpages.com/hub/how-long-to-knead-with-stand-mixer
Have fun and enjoy experimenting with the texture of your bread dough while kneading! :)
I agree that each dough is different but I usually let the just-mixed dough (shag stage) sit for 5 minutes before working it into a ball and then knead a one-loaf country loaf of dough by hand for about 8 minutes. (And I put the timer on for 8 minutes to be sure I reach at least the 8-minute mark, going on a couple of minutes more as needed.) Most dough seems to transform to softer and more supple by 6 or 7 minutes into the kneading work and then feels "alive" and soft by 8 minutes.