How do I keep Low Power Mode permanently on my Series 7 apple watch

According to Use Low Power Mode on your Apple Watch, the longest period of time available is 3 days.

"Each time you turn on Low Power Mode, an information screen appears. Scroll to the bottom, then tap Turn On or Turn On For. If you select Turn On For, you can choose On for 1 Day, On for 2 Days, or On for 3 Days."

Peptide Maven wrote:

Thanks for the suggestion. Any way to keep low power mode indefinitely?

That is not one of the options.

You can let Apple know it's something you'd like to see here:

Product Feedback - Apple