How long my acne medication would take to show results

Patient: Hi m 21 year old i have been suffering from acne for the last 3 years.I went to doctor this year for the first two weeks hi has given me antibiotics that did not do well then he put me on isotretinoin.I have been on medication for the last 17 days but it does not show any results m still getting acne .my question is how long my medication would take to show some results.