It depends on what type of medication you are prescribed.
The most common medications used to treat bipolar disorder are mood stabilizers. There are many different types of mood stabilizers, and each works a bit differently… But generally, it takes at least 2 weeks for them to take effect. If you are hospitalized for a severe manic episode, a psychiatrist might temporarily give you a higher dose to help you get back on your feet more quickly.
Sometimes people with bipolar disorder are also prescribed antidepressants. These generally take 4-6 weeks to take effect. Antidepressants only help with depression, not with manic episodes—so you might not notice their benefits right away unless you are having a depressive episode.
People with bipolar disorder can also be prescribed anti-anxiety medications, like benzodiazepines. These work very quickly (30-60 minutes), and wear off after a few hours.
Every medication is different, and they all affect each person differently. The doctor who prescribes your medications can give you more detailed information about the specific medication(s) you are given.