I also use the Flush King and am very partial to it... but then I don't have any SewerSolution/other experience to compare to.
As to flush time - I have found it to vary quite a bit according to who is using the facilities, and for how long we have been on site. This is at full hook up locations - NOT boondocking. The Flush King, with the 'see thru' section aids mightily in this endavour. If the wife/daughter combo are along, the TP is what causes the most 'need' for flushing. I keep refilling the tanks until the water - ALL OF IT - comes out as clean as it went in. Then I usually do one more 2/3 full flush of the BT, just to be sure.... and darned if on 2 occations, there wasn't more that needed to get out
Can't seem to get the girls trained up, as of yet...
As far as actual time it takes, I can say that it has been as few as about 10 minutes - to upwards of 45 or so....Water pressure is likely the #2 variable in this process, #1 is noted above. But for me, KNOWING that the BT is clean is important to me, so I take as long as it takes.... Heck, I paid for the water & sewer, and by gosh.... I'll use it, thank you very much!
If there is any question as to your BT cleanliness, add about 5 gallons or so, then add a bag or 2 of ice cubes - pull to the next CG, dump... and prepare to be amazed! at what comes outta there...
All the best, and ENJOY that AirStream!