What you mixed up has spores, dormant disease organisms, that will not stay viable very long which is why most every sprayer comes with directions to discard unused mixes and clean the sprayer really well after use.
The bottle of concentrate does not mention that since the manufacturers, or packagers, assumption is that you will use all of the mix or discard it immediately if not.

Thanks kimmsr. Its been in solution for 3.5 days by now so I'm guessing that's too long. I'll discard it and mix a half bottle for future applications.

That brings me to another question, about the bottle of concentrate. For how long do those contents stay effective once the foil seal is removed? No mention of that either on the (small) bottle but it contains too much concentrate for the typical home gardener to use up in one application.

Call the 1.800 number on your container Bt to find out how long the product will remain viable once mixed, as well as the storage life once opened. I expect that if stored in a relatively climate controlled environment, that your original container will be good for quite a long time. That means out of the sunlight, and not exposed to very warm or cold temperatures.

Just so you know, MOST pesticidal products (insecticides, herbicides, and fungicides, etc.) begin to lose their potency not too long after they are mixed with water. Mix only what you think you will actually use in one application so that there is nothing to store.