That depends on far too many issues to give an exact time line.
1. It varies by person
2. It depends on how high the level went
3. It depends on what can be done to treat it
4. Was it bad enough to need emergency medical treatment at a hosptial
1. Each person is different and will respond to treatment slightly differently. Two people can have the exact same high sugar reading and do the exact same thing to try to bring it down, but one person may have better results than the other person because no two people are going to react to treatment in exactly the same way.
2. "high" blood sugar can be anything from 200 to 1000. So, just saying how long to recover from "high" sugar is too "wide" of a range to give a response.
3. The treatment for high sugar is going to vary by person and situation. A type 1 diabetic will usually just take additional insulin and usually feel better in a short time after doing so. Insulin will bring sugar levels back to normal range fairly quickly. A type 2 diabetic might not have the option of using additional insulin and would have to exercise, drink more water, and watch what they eat. It could take several hours for a type 2 diabetic to feel better and have a more normal sugar level after their sugar levels have went too high.
4. If sugar levels go high enough, then a trip to the hospital might be required to regain control of the levels. How long the recovery would be depends on how high the levels actually went and if the person had ended up in a coma due to the high levels. So, recovery from extreme high levels could be a few hours or several days if a coma had happened.
So, final answer - as little as 30 minutes if insulin is used to bring it down or as long as a few days if a diabetic coma occurred.