Are there any circumstances where schizophrenia or bipolar disorder are not lifelong?
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