“How long does it take for brain chemistry to return to normal after antidepressants?”

The assumption behind this question is that depression is caused by disturbances in brain chemistry. This was a very popular idea, but if you look at the research there is nothing to back it up. Antidepressants do not normalize brain chemistry. In carefully controlled studies there is no evidence that they are more effective than a placebo. In some situations people will feel the side effects of antidepressants. This can be interpreted that something is working and they may feel better. But it's not brain chemistry.

The class of antidepressants is a very big category and within this class are drugs that have tremendously different duration of action and clearance from the body. So as such, this question cannot be answered because it is not specific, The other problem is the word, "normal." Antidepressants are supposed to get the brain to work close to normal. Without treatment, the brain chemistry will be abnormal.