When will my sourdough starter peak?

Pick the ratio you fed with and when you fed it. The timing comes from King Arthur Baking's tests at about 78°F (26°C).

Ratio

Fed at

Peak · 1:1:1

Peak expected around 12:00 PM–2:00 PM

King Arthur Baking: a 1:1:1 feeding usually peaks after about 4 to 6 hours.

A peak is a window, not a moment, so this shows about an hour either side.

These times are for a warm spot, about 78°F (26°C). Warmer is faster and cooler is slower, so note when yours actually peaks a few times and adjust.
A starter that has risen and then fallen back was at its best a little while ago and is getting hungry.
Sourdough Starter Tracker shows this after every feeding and sharpens it each time you tell it your jar is at its peak.

Sources