@qaccy
I'm going to let someone else elaborate on how the slots work but I wanted to mention the schedule. The recipes switch every six hours (so, four recipe sets a day). You can also use diamonds to switch at will, as opposed to waiting until the clock finishes counting down to the switch (resetting the countdown to six hours from the time of diamond use).
I want to clarify part of the statement by
@mikeledo about the time instants. Yes, you can use time instants to make sure the craft finishes before the switch, but most important is the
collection. Even if it finishes before the switch, but it isn't collected, it will still stall that slot. The other four slots will change but that one won't (until it is collected, it isn't available to receive a new recipe option). Each of the five slots carries different categories (for lack of better term). It isn't first slot, second slot, third slot, etc. The slots will appear in different orders but there are things that always appear in the same slot as other things (I don't have it written down and I'm too tired to recall specifically, but hopefully you get the idea or someone else can explain better).
Pet food is available in the same slot as relics, so if you craft relics and fail to collect before the next switch, you may be missing out on an opportunity for pet food (or a different relic you may want). Same if you craft pet food and don't collect in time, you will miss opportunity for possible back-to-back pet food. When you select a recipe to craft, it tells you how long it takes and there is also the countdown timer showing how long until next switch. Use time booster if necessary to ensure the craft finishes first, but also make sure you collect. Personally, I set an alarm on my phone in case time slips away from me (sometimes I just set it for five minutes before next switch so I can collect whatever I crafted, and then hang around a few minutes and see what my next choices are to begin one and then move on with my day).
As far as frequency, I've never tracked the pattern. I know there are periods when I'll see something frequently, and periods where I won't see it for awhile. I recall one time in particular it took a few weeks to craft magic dust relics (and I needed them for CCs) and it was frustrating. Now, I always craft pet food, just in case, and often craft relics if they're my CC ingredients because I use those the most and never want to run out (I have all five queue slots for CCs near perpetually).
There are four opportunities a day (not counting diamond usage, which I don't do but it is an option) to find what you want and some days are luckier than others. Just the way it is. But understanding the schedule and having a plan will help you maximize opportunity and minimize missing out.
Sorry this got long. I may have gotten carried away... but I hope this helps.