do you mean how many rows before you decrease?
it would depend on the size of the hat.
here is a good chart for hat sizes:
It really depends on how you are decreasing, but I like to start my decrease rounds about 1-2 inches from my final height needed (less rounds to decrease a baby hat, more for an adult).
with 64 stitches I would typically decrease starting every 8 stitches (then knit a row, next row would decrease ever 7, and so forth) but again that would vary depending on weight of yarn
what size are you aiming for?
sorry... i probably created more questions than i answered