As I have understood it, water temp is key. I usually restart mine when it reaches 50°F. And I don't go gradual, I just keep it on for the rest of the season. This is for Iowa/'up north'. If you can, monitor your water temps and that may influence when you restart.
Just my 2 cents. I'm sure one the limnologists can chime in.

"Politics": derived from 'poly' meaning many, and 'tics' meaning 'blood sucking parasites'.