How Many Grams Of Pseudoephedrine HCI Can I Purchase In

How many grams of pseudoephedrine HCI can I purchase in a 30 day period? I purchased a 15 count box of Allegra D (3.6 grams) on 3/30/15 and again on 4/10/15. I need to get a 14 day supply before I leave for an out of the country trip on 4/30/15. Will I be able to purchase another 15 count by 4/29/15? As of now, I only have 8 pills left at home. I suffer with terrible allergies and don't want to travel without this medication.
Thank you