Colder temperatures tend to keep compounds fresher longer once mixed
Bac Water by Manufacturing process is only "good" for 28 days. This is why you hear the 30 day rule very commonly. Which is a mis perception.
The fact is the compounds will still be active and work, even though over time they will lose potency. But the thing is the Sterilization is only good for 28 days. So after that you are risking bacteria, etc, in the mixture. It's really not a huge deal if you are shooting SubQ but def can be an issue if you are shooting IM.
I have done mixed HCG a year later that sat out the entire time and I assure you that it still worked.
Theres really no studies to prove just how much potency each compound loses once mixed over time.