H.V.M.N. Ketone Ester will elevate your ketone levels for up ~4 hours.
The length of time will depend on if you exercise (where BHB will be used quicker) and what you eat (where BHB will stay up for longer if you are fasting or following a ketogenic diet).
During exercise, the effects typically last ~1.5-2 hours.
At rest, the effects typically last ~3-4 hours.