With 20k, I only added once, about 10.00 worth at Flying J, truck stop. I tow trailers 6000 lbs and my soot built up doesn't increase, in fact it improves. A hot engine slows down the soot built up and it normal to go 300 to 600 km without a REGEN. That happen often for me. REGEN in the city is about 90 km, which I do almost daily. During winter it will drop and drop a lot if you don't have winter covers.
Most of my mileage is city driving, averaging between 9 to 11 k/pr 100. Highway, I hit 9.2 several times and that driving in the rockies. Doing 10 percent over the speed limit. 120KM per hour.
The key is to warm up truck asap. (you need, it is a must, to use winter covers on the grill when 7c and colder) This decreases def use and instead of driving 20km before truck warm up, it warms up in 10km. It also improves fuel economy. FCA will give you one new winter cover for free. I got mine for free.
If the truck doesn't warm up, even when filter is at 65 percent or more, it will not do a REGEN. EGT3 and EGT4 need to hit around 1200 F and your EGC, coolant around 160 f or so. At about 80 percent, you will get a EVIC message saying the filter is 80 percent full and to drive the truck ( you can idle at 1800 rpm or so). If it hit about 90 percent, you will be force to bring it to dealer.
Neither of those are convenient, nor fuel good for fuel economy and this shouldn't ever happen. This is where having the EDGE CS2 monitor comes in handy. You can initiate a REGEN via this device, though I never used that yet. I monitor and have alarms set for EGT1, EGT2, EGT3, EGT4 and ECT. With the winter covers, I have the alarm set for 230 degrees F. Then I open up the winter covers. Soon, I be removing it, as spring is approaching.
After many km of driving with soot built-up and using cheap diesel gas, your injectors will carbon up, resulting in poor fuel economy and sluggish performance. The engine sound more diesel noise. See the video from shell on it, I do have it posted on another ecodiesel forum.
I fixed that issue, by using only Shell V-power diesel, about 8 cent more, after driving 17,000 km with the cheapest diesel, about the second fill up, when I reached almost 1/4 empty, I suddenly notice a big difference. No hesitation, more pick up and smooth acceleration and quieter sound. Starting up, in the coldest days in Calgary, it was instant. There are additives too, but I don't want to carry bottles and it cheaper to fill up with Shell V-power diesel. I think in the US, the call it Diesel FIT.
Such a big difference, I have filled up a third tank.
If you do very short drives (20km or less) and it cold and you don't have winter covers, then the soot will climb about 1 percent every 2 or 3 km. You have no option but to switch out vehicles or invest in a monitor device, so that you can time things, so that you don't get 80 percent or 90 percent EVIC message.
hope this helps, I normally not on this EcoDiesel forum.