Fermentation is finished when it ceases to off gas. The airlock is still and has reached equilibrium. If you brew in glass, look at the beer, the yeast ceases swimming and flocculates (settles) on the bottom.

What is complete fermentation?

Completion of fermentation process indicates low concentration of glucose i.e. 0.2 g/Lit. The completion process depends on the initial glucose content, pH, nitrogen assimilation, Dissolved oxygen etc.

When should I finish fermentation?

When is the Fermentation Over? The fermentation is considered done when you either reach your desired sugar level or go "dry" at 0° Brix. A wine with 0.2% residual sugar contains two grams of sugar in a liter of wine.