The time depends on whether you are smoking Baby Backs or Spare Ribs (or St. Louis cut). Spares typically take about an hour longer in my experience.
I don't think my #3 runs as hot as Brian's. I may have to do the knob adjustment suggestion to try to adjust my temps in my #3. So, I always leave both baby backs and spares in for 5 hours minimum and usually have to go to 6.0-7.0 on Spares and up to 6.0 on Baby Backs.
My #3 racks are so big that it is hard to clean stuck on stuff. So, I typically take mine out when they are done and then sauce and put them on a cookie sheet and place under my oven broiler for a few minutes to heat up and caramelize the sauce. The cookie sheets are way easier to clean up than the smoker grates.
I really prefer the mustard as I think you get a better bark than with oil. But in a pinch, oil is definitely an acceptable substitute.
Regarding resting, I typically don't bother with ribs. I don't think it is really necessary. If it were really that important, I would think that moisture would start gushing out of my ribs when I cut them without resting and it doesn't. Now for Pork Butts and Briskets it is VERY important. If you start to pull or slice a pork butt or a brisket without resting you will see all of the moisture just gush out of the meat which is not a good thing. That is my two cents on resting.