It largely depends on your diet pre-fast. If you ate a standard American diet (moderate to high carb) with a significant amount of sugar and/or processed foods, or can take up to a few days to get into ketosis.
For me, i need 2 days to get into ketosis.Or 4hour, if i go a bike ride fasted, but that is a hooorible experience, would not recommend :D
When an app says that in 12h you are in ketosis i always laugh. If you eat 3000kcal meal, in 12h that food wont even leave your system…