The Degradation of Limewater in Air by Randy Holmes-Farley - Reefkeeping.com
6. Precipitation of phosphate in limewater and in the aquarium by Craig Bingman, Aquarium Frontiers, Fall 1995
see also: Limits To Limewater...Revisited by Craig Bingman, Aquarium Frontiers, August 1999
In the amount of time it takes a common hobbyist to unload a batch of 20g of kalk mix, it will have lost very little in potency. I do this with 50g's and use it every 3 weeks with a system I run. I've tested before, and at 3 weeks, it looses very little. YMMV