I always wait to add DEF until I can be confident of using up at least one 2.5 gallon container. I would not add a second container if there was any question about the tank being low enough to hold that much.I do you all just toss the unused portion?? I have yet to see an expiration date...
Some of the date codes are less than obvious and I have avoided those brands. If your curious, I found some You Tube videos that helped to interpret at least some. Here is one example: