Brisket injection timing ?

I did a search here for injecting brisket, and while there are those that do and those that don't, since I have never injected, I am going to give it a try.

I have a flat and will inject with a 50/50 mix of beef broth and vegetable stock.

My question for those who inject, would I do it tonight and let it sit til morning, or do it right before, or does it even matter?

I will also be smoking in the 300* range.