If I had to guess based on my own experience, I'd say about 25ml of menthol crystals to an ounce.
I came up with this because I filled a 60ml dropper bottle with 50ml of PG and 1/2oz of menthol crystals. It almost overflowed the bottle. Keep in mind, this is diluted, but it should still be about right.
Since 1 ounce is about 30ml, the answer is "Probably about even as long as you crush it finely"
