This can vary a great deal between patients. Most patients who respond to ketamine find that a single infusion will provide at least several days of symptomatic relief. This means relief of the physical
symptoms that make depression/bipolar/PTSD so unbearable: anxiety, anhedonia, physical fatigue, dysphoria, cognitive impairment, insomnia, etc. Patients who get a series of multiple infusions over several days often get symptomatic relief that lasts weeks.
But there’s more to it than that. When the physical symptoms are relieved, that can also trigger a dramatic improvement in mood. It can help patients feel healthy, function normally, and stop feeling negatively about themselves, sometimes for the first time in decades. Even if the physical symptoms begin to return, many patients find they can withstand them with more resilience than before, instead of retreating to the fetal position each time a new stress or obstacle arises. This improvement in mood and function can last longer than the physical symptom relief. See here for a more complete explanation.
The number of patients using ketamine long-term is still very small, and long-term treatment regimens are still being developed.