Before the peak
Everything climbs, peaks, then declines. Cultures included. Charles Handy called it the Sigmoid Curve.
The trick is to start positively changing before you hit peak performance, while you still have energy and success behind you, rather than once results start to dip.
Most leaders start the culture change process at the latter point, by which time it is already too late. The decline has already set in and consequently recovery is far harder. You need to change before the peak.
To fully understand how this model works, you can download my free guide on how to make culture change stick at www.colindellis.com/culture-papers.