Depends what you are looking for. If you want to know what your “actual” numbers are, you should only fast as long as they normally tell you (10-14 hrs). If you are trying to hit some artificial goal number (set by a doctor, employer, insurance, etc.) then fast however long you want. If you are trying to drop your cholesterol number specifically, you may want to look at www.cholesterolcode.com for some info about how this number does not truly correlate with good CV health, and for a way to drop it without fasting.
And for what it’s worth, I did a full blood screen when I was 7 days fasted last summer and my blood sugar was around 75 (my normal is \~100), Total Cholesterol was 137 (vs \~220), LDL was 70 (vs \~140), HDL 38 (vs \~40), and TriG was 146 (vs \~240). My BP was also a little lower than normal (112/72 vs around 124/80). I had done several multi-day fasts in the weeks leading up to that, too, but it wasn’t consistent. Note that fasting, even for a week or two, will not significantly change your HbA1c number (one of the indicators for Type 2 Diabetes) because that number depends on your average blood sugar over the past 3 months.