How long do I have to correct a cal mag deficiency?

If I ever begin exhibiting problems related to a cal mag deficiency, how long do I have to steer the ship right before she sinks?

Would sprinkling dolomite lime on the top soil and watering it in do well to solve the problem?

Mix a table spoon of Epsom salts in a gallon jug mist the plants and they will tell you next morning. wont hurt either way.