21 days
 

Your body needs at least three weeks to adapt to dietary modifications, so be patient. Don't anticipate immediate changes. If you start a high-carb, moderate-protein, low-fat diet with reduced calories, visible progress will probably not occur until after 21 days.

Useful info, eazy, as far as I know 21 days are also needed to develop new habbit, like writing each morning a blog post or doing cardio 3 times per week in the morning.