I try to stick to the one pound per gallon rule, but you can have more or less depending on what you like. Some people like deep substrate, some don't. I have 120 Lbs. of African sand in my 120 gallon.
Can't really use a gallon rule being that some tanks are taller and the footprint could be smaller..So you figure there is 4 square feet in a 55 gallon tank.. I heard a 50 lb bag of sand will cover 6 square feet at a total depth of 1 inch.. So a 50lb bag should give you about 1 1/8 depth in a 55 gallon tank given your measurements are 4 ft x 1 ft..