Hi,
Honest answer - it varies. A lot.
It depends on how often you have been in ketosis before. The more often, the easier it seems to slip back into ketosis.
Also, the stricter you low carb, the quicker.
Also, different people, with different bodies, activity levels, glycogen stores, and metabolisms get there at different rates.
Have you got any ketostix?
My experience is I usually feel the first indications after 3-5 days, sometimes less. And from that point I am firmly 'in ketosis'. But the proper ketoadaptation, where the whole body has adjusted, stamina is up, BG is stabilised, sleep is deeper and refreshing, cravings a distant memory, glycogen stores minimal... That takes weeks.