Hi there - it depends on your particular situation! If you feel like you already understand much of the content (or it at least feels familiar), then you can start doing them fairly early. The problem is that AAMC doesn't split up their Qpack passages by topic. This makes it kind of tough (in my opinion) to use the Qpacks as an assessment, especially since a good deal of the questions are "AAMC reasoning-based" and not dependent on a particular piece of content (or dependent on more than one). An ideal assessment tells you which areas you need to really focus on and which are already strong, and AAMC materials aren't necessarily the best option for doing that, though they're awesome in other ways. Taking one or two good full-lengths will tell you more about which content areas are lacking than much of the Qpacks would.
Instead, I generally advise working the Qpacks into your last 1-2 months of prep. This way, you've at least seen the content once, and questions you miss are more likely to be due to logic or strategy issues that will then be easier to pinpoint. It's also generally a good idea to see more "test-like" (in other words, AAMC) materials closer to your test date. Otherwise, you run the risk of going through all your AAMC resources (except the full-lengths) too early, and then being stuck with less representative practice.
In any case, no matter how you work these in, they'll be helpful as long as you split them up and review them intensely. Some people like to take the Qpacks / Section Banks in large chunks, similar in length to one section of an FL; I personally prefer to divide them into as small of chunks as possible, which works out to only a couple of passages per "attempt." This way, as soon as you finish a chunk, you can jump right into careful review. I recommend keeping a "why I missed it" log, which is always helpful, but more so with AAMC resources.
Anyway, good luck 🙂
