Start by reaching out to your parents' financial institutions now and notifying them of your power of attorney status. If you can go with your parents to their bank (and other financial institutions) to show the bank that POA document, even better. The benefit of doing this now is that your parents can sign any documents their financial institutions might require. And it will make it easier for you to step in and start acting as their POA when the time comes.
So when is the right time to start using your power? Not until you absolutely have to. Let your parents maintain their independence as long as possible, but make sure you're monitoring their finances to ensure they aren't making costly mistakes. Ask if they'd be willing to use the Carefull app and link their accounts so you can be a second set of eyes for them. The app will provide you with view-only access and will send you alerts when it spots missed payments, duplicate transactions, changes in spending and many more issues.
By getting that sneak peak into their daily finances, you'll be much more prepared once you do have to start using your power as their power of attorney to make financial transactions and decisions for them once they're no longer able to themselves because of health issues or cognitive decline.