The temperature of the solution seems to have an effect. The warmer it is the faster it seems to work. I've had parts submerged soaking in it for 24-36 hours when warm(the shackle bracket above was 24-36 hrs) 48-72 hours or so when cool. I dont bother trying to spike the mix to rejuvinate it. Its so inexpensive I just dump it and make a fresh batch. If you take the time to thouroughly clean the part of dirt, grease, undercoating, etc. it cleans up faster and the solution lasts longer. I've had the paint removed by the mix on some parts like fender braces.
It doesnt seem to "go bad" over time, just with use. I've used a 3-4 gallon batch for dozens of parts; spring hangers, shackles and brackets, nuts, bolts, exhaust hangers, etc. over the course of 9 months and it worked well.
You will likely see sediment forming in the bottom of the bucket with a lot of use.