When are you applying to school? Whenever it is, make sure you take the test by September of that year (preferably June).
You should dedicate as much time as you need to get the score you want. What's even more is making sure your study time is QUALITY time. Most people don't study properly (constantly timing themselves, not reviewing anything, no redoing or reredoing anything, allowing distractions, etc..) and they waste hours and hours of valuable time they could have used more efficiently.
Starting at 168 is good news. If you're shooting for T14, use all the resources at your disposal to get that up as much as you can.