Factors affecting the rate at which it lasts also include weather conditions: salt blocks tend to dissolve faster during hot/rainy seasons when cattle take in higher water (with salt intake also increased).
How long does one block of salt last for cattle?
Salt blocks for cattle should not have a limited "lifespan" as such because they dissolve gradually depending on how much the animals lick to meet their sodium requirements; it can last several months or even up to a year before it completely dissolves or needs replacing when it gets too small for their convenience.