You have 6 months to bank up to 100% of your points - so with a March Use Year you can bank up to 150 points thru the end of August. You can bank up to 50% thru the end of November and up to 25% thru the end of December. In order to bank your 48 points you need to call MS before the end of November.
If there is any doubt, just bank them right now. If you later decide to travel before next March, you can borrow from your 2008 Use Year.
Enjoy!