Even if they spend all day licking it, their body will prompt them to take on enough water to counteract the salt, and the excess will be excreted, so no need to worry.
My home-bred filly had a salt lick from the first week of her life. She loved them as a baby and I was a bit concerned at how much time she spent on them but I soon realised that salt is not automatically bad for them like it is for us