Minimum of 2 inches... 4 is better. Be sure to tamp it down when it's damp. A compactor designed for this gives the best results, but unless your brother-in-law is a paving contractor you're likely going to have to rent one and they are quite heavy (and expensive). So, use a piece of wood about 10 inches square nailed to the end of anything handy to make a handle... spare broom handle, kited rake handle or borrow Uncle Morris' cane...(just tell him it's for a good cause... you'll have it back to him in a jiffy).
Best of luck!