Who determines when "Springing Durable POA" takes affect?

The answer to your question is really "it depends"!! Most POA documents have a statement that indicates the criteria for activation, often when two physicians certify that the individual is incapacitated. I recommend you review your document, your answer may be right there!
PS. Good luck on your upcoming Alz journey, take good care of yourself and learn all you can. Hang in there.