You are supposed to throw it in the trash and buy a refractometer instead. :)
I don't know anyone that lets the hydrometer sit for 24 hours. Most people just dip it in the tank and take an instant reading. They are very innacurate though, even right out of the box, a half dozen brand new ones will give you a half dozen different readings with a very wide spread. And they need to be replaced every 6 months.
You are better off spending the $30 to $40 and getting an accurate and reliable refractometer.