Question about Weekly Shipments stock

So for the last few weeks, Weekly Shipments (at least for me) contained Clone Wars stuff and Raid Character Shards (and Negotiator blueprints), and I bought the 5 Traya Shards every week. Starting from last Friday, however, the stock changed to being Imperial-themed. How often does stock change, and is it always different each time, or will it eventually cycle back and restart?