Are there any circumstances where schizophrenia or bipolar disorder are not lifelong? It is known that psychiatric disorders such as schizophrenia and bipolar I, and bipolar II are for life, and require long term treatment (lifelong).

Is this always the case?

The answer could include reference to any disorder, even those not listed in the DSM, say bipolar III, bipolar IV, bipolar V, etc. This refers to what is known as bipolar spectrum disorder.
The current understanding, I believe, is that symptomatic psychiatric disorders may or may not be lifelong according to many factors, but that the susceptibility to them probably is lifelong. Some examples. . .


Substance-induced psychosis (which is pretty much what it sounds like) is often though not always temporary.
Certain types and locations of brain tumors may cause psychotic symptoms, which may or may not reverse on treatment.
Afr Health Sci. Dec 2004; 4(3): 190–194.
Major depressive disorder has a highly variable course, and may reoccur or may not.
N Engl J Med 2008; 358:55-68. DOI: 10.1056/NEJMra073096