approx how long for brat sized german sausage?

no not at those temps my sausage takes 6-8 hours but i do lower temps then this

BOX? my sausage dosn't come in a box.and mine takes lots longer too-low & slow.

Do you have a meat thermometer? If not, I suggest you get one as well as a separate one to monitor the temp of your smoker. A good digital with 2 probes, one for each application can be had on ebay as well as Gander Mountain and Cabelas for a small price. Back to your temps. When I made my brauts I did this. Place your brauts in a PRE HEATED smoker at 130 degrees with your dampers wide open for and hour or until the casings are dry. After that, adjust dampers to 1/4 open and gradually increase the temps of your smoker to 165 degrees and hold at that temp until until an internal temp of 152-155 degrees is reached. After smoking/cooking is completed and the brauts reach the disired internal temp, remove and give them a shower of cold water until the internal temp hits 110 degrees. The cold water keeps the sausage from shriveling up. You can also do it by filling a 5 gal bucket with ice cold water and dunking them in it. Hope that shed a bit of light on it. As far as your smoke goes, youll just have to experiment with the size of wood you have but I think a safe starting point would be smoke for 1/2-1 hour, not really heavy smoke, light and thin. Remember, when smoking, its low and slow. Go by the internal temp, not a watch. And as you smoke more and more and get used to your smoker, you will able to judge how long its going to take for something to get done. Good luck.

the 120 for an hour is to dry out the casings prior to smoke so they actually take the smoke

The 160 for 30min is to heat them up so they take a good smoke and pass the danger zone quickly

The 180 til 156 internal temp is to finish cooking them

I'm assuming when you say box and based on the temp schedule you used some sort of seasoning kit? These kits give directions to reduce liability which is why they finished so fast. It's not necessarily a bad thing, but too hot too fast can render the fat out and make a bad texture.

Most guys on here will loosely follow the same temp schedule, but its done from around 120-180 over 6-8 hours upping the temp 5 or so degress every half hour.