The amount of time can really vary by smoker and how much you are smoking at a time. Also, how much moisture is in your sausage as well as whether you hung your sausage overnight to dry. But, for the most part I leave the dryer on the whole time after the smoking phase is over. But, I am usually dealing with a packed smoker with tons of moisture. You may find that you have to pull your dryer off a little earlier to fit your liking.
Another example is from a couple of weeks ago. I made 4 batches of sausage with two batches that fit in the smoker at the same time (this was in a homemade fridge smoker). One of the four batches was my Krautwurst which was just regular ring sausage with sauerkraut added.
The first smoke was the two regular batches and they both finished at the same time, which was probably 4-6 hours or so.
The second batch was comprised of one regular and then the one Krautwurst. The Krautwurst was so much more moist that it took double the time of the regular. I learned from this that in the future, I am not going to combine my batches like this.
I did not have time to hang any of these sausages overnight, so a good 1/3 to 1/2 of the smoke time was at low temp trying to dry them out a bit before beginning the actual smoke phase.
In my Smokin-It #3, I usually use about 3-4 ounces of wood, and have to rotate my sausages several times during the smoke to maintain even heat. If you did not rotate your sausages, this could explain why you had tougher parts.
I probably do a full rotate front to back and top to bottom at least 3 times per smoke and then as some sausages get done, I am pulling them out and do some manual rotation on any that remain. My #3 definitely runs hotter in the rear, so the rotation is really necessary to keep even heat.
For the most part I only bother with rotation when I have full racks of sausage, snack sticks, or jerky. But, for larger cuts of meat like pork butts, briskets, and ribs, I don't bother rotating.
But, knowing that the rear of my smoker runs hotter, I do always place the thicker part of meats that vary in size such as ribs and brisket toward the back of the smoker. So, the the thicker/fattier point of my briskets are always facing the back leaving the thinner/less fatty flat pointing toward the front of the smoker. The same goes with ribs. Depending on the cut of ribs that you are using, there is usually a thicker end that I place in the rear of my smoker.
Hopefully this helps.