Postgresql behavior at end of sequence
If you try it, you'll get:

ERROR: nextval: reached maximum value of sequence "testseq" (9223372036854775807): select nextval('testseq')


This is the case when the sequence is created automatically for a serial or bigserial column.  You can create a sequence with specifying CYCLE, in which case it will restart from the MINVALUE specified (or left at the default 1).