Well I go through about 1 a month. Last year I did 1 every 2 months. None got hard, or even close. The trick is to keep 0 moisture. Do not leave the scoop in it and do not get the scoop wet when using it. Keep it sealed, cool and dry and it will last. However, I would not suggest it as... well... basically, shit happens. In smaller set ups, I would suggest you get the box if you want to buy bulk and save. It has 4x50 gallon mix bags inside of it. Much better choice for you so long as you remember to seal the bag closed in between.
A bucket of salt easily last over a year if you aren't negligent.
The bucket is sealed by a big O-ring.
As you say obviously don't put anything wet in there, dry measuring scoops can be left in there without a problem.