A general rule of thumb (for me) is wait 15 minutes if the resin is about 75 degrees after I mix in the catalyst. And several things make a difference. The temp in your pouring area will affect the timing, along with the age of the catalyst, how much color you add. Is the color dye or mica powder. Some blues take longer than others. There is a real learning curve that goes with pouring. You actually get a feel for the resin, and when to pour. And I've found if I haven't poured in more than a month, It takes a batch or 2 before I get the "FEEL" back.
I know that doesn't really answer your question. Plus yellow and blue aren't the easiest to start with. One of the best blank makers on this site told me it takes pouring about a gallon of resin before you start to get a feel for when to pour. And that was pretty close to my experience. My best advice is to start with small pours to limit the expense of your learning. And no matter your experience level, you'll get some really nice pours in that 1st gallon. The real test is repeating your successful pours.
I made a blank for someone that I really didn't expect to turn out. He loved it, and so far I can't get back to that combination. So take good notes.
