Depends. Some can study for the OAT in 1-2 months. Others take more than 2 months. Depends on your situation and when you start studying for the exam. Also how well you remember and understand concepts from previous undergrad courses that are being tested on the OAT can also have an impact on how long you need to study for the OAT. But I'd say 3-4 months should be enough. Anything longer than that can be overkill for you mentally. In my case, I studied for the exam from August to December.