According to plumbing code, drain pipe has to be sloped a minimum of 1/4-inch per foot and a maximum of three inches per foot. A slope of less than 1/4-inch per foot will cause constant drain clogs and a slope of more than three inches will allow the water to drain without the solids.
If you are need a drop greater than this, it is best to use 2 - 45º elbows to create the drop to a lower level instead of 2 - 90º elbows so that the solids do not create a problem when dropping vertically.