Hello Dan,

I don't know when you plan to pour, but the conventional wisdom I've heard is that concrete increase its strength indefinately until its first freeze. I've seen young concrete from days old to even a couple of weeks that seemed to be inferior (from powder to just plain soft) to those that had cured a while. The other thing to remember is that concrete curing is a chemical reaction, so the colder it is (above freezing) the longer it takes.
For low walls, blue board on the sides (if backfilling this is permanent) and top insulates and lets ground heat keep your walls warmer longer.
Blue board is expensive. If you're looking for lots of the temporary (or permanent) kind, call a few industrial roofing contractors now, they often pitch board from reroofing projects. I've seen up to 6" board hauled to the dump.
I'm a bit of a perfectionist, but I don't like seeing anything go under a month before its first mild freeze. If you're in a pinch you can play with the chemistry, which is a fine art and science.
A good source of information is your local batch plant. That's where I take my technical questions.
Best of luck,

Zach