Darben,

First welcome to FT!

To answer the exact question you ask, most promo codes come out early in the year (or last month of the previous year) for the rest of the year. So look around here in December or January for new promo codes. That said they can come any time even in the middle of the year.

On a broader issue when it comes to Amtrak, you should always book ASAP with the best discount you have at the time. As long as you haven't picked up the ticket you can cancel it at anytime even after the train leaves for a full refund (as long as it isn't a sleeper). If a better discount comes out you can try and use it. However with the fare bucket system it may not be worth it. For example lets say todays fare is $100 and you get 10% off with AAA making it $90. Next month you find a promo code good for 20% off, but now the full fare is $120 so after the 20% off you're paying $96 which is $6 more than what you had booked originally.

While I know it wasn't the answer you are looking for, I hope it helps.