I wondered about this after seeing your post and long story short, descalers actually create a chemical reaction that changes the scale to another chemical altogether. Looks like it depends on what kind of descalers you use. Vinegar will react to scale differently than a citric acid based descaler.
But limescale and is soluble. So it will dissolve in water (especially cold water). Given that I would imagine it's not the best idea to reuse a deacaling solution more than maybe twice.