Best thing to do is:
• Forget the idea that there's a certain number of months ahead of your expected travel when the lowest buckets will be available. In my extensive experience they can be made available at
any time - tomorrow, this month, next month, 5 months out or 11 months out.
• Next, use AmSnag...
http://biketrain.x10.mx/amsnag2.0/amSnag.php ...to make searches in increments of 30 days starting tomorrow in order to find and record all 5 buckets for the sleeping accommodations you might want to get and for the end points you will use. (Be advised you may not be able to find all 5 buckets even if you make 11 months worth of searches)
• Once you know what the buckets are, you can see if they're available when you want to go:
▲ If low bucket fares are available when you want to go simply book it - all they can do if you wait is increase or become sold out!
▲ If low bucket fares are NOT available when you want to go, you can either:
♦ book now and hope the fares decrease. If they do, call and have your reservation modified to the lower fare or
♦ Wait and see
if the fare decreases, and if you'be got a good Fairy Godmother the decrease will be to the low bucket. However...
♦ The fare may never decrease or it may increase or become sold out.
I'm firmly convinced the only hard and fast rule about the best time to look for low bucket fares or when the buckets change is that there are no hard and fast rules. I've seen some routes with lots of low bucket sleeping accommodations scattered out among the 11 month availability period - and other routes that NEVER have low bucket sleeping accommodations offered.
I've heard here that bucket changes are all made more or less simultaneously at 2am PST when they're programmed into Arrow, the Amtrak computer. However, my experience (based on receipt of 34 Fare Watch emails from AmSnag) shows bucket changes - at least the notifications of them - can occur at any time of the day.