The specific gravity has not changed-the water did not run out of the slip or evaporate into thin air. A thick condition is caused by a chemical change from the sulfates. But nobody ever told you that, so you add water and that may ruin your slip. That's nearly the impossible to say exactly. It's going to vary with the situation.
Time is a factor.
The environment is a factor. The original formula is a factor. Proper mixing is a factor. Turn the mixer on and get the agitation going. This in itself will lower the viscosity and make the slip more fluid.