postgres process and ps stats

During the execution of program entring data in a pg DB I follow what
the postmaster is doing using ps. From ps's output it seems that the
process uses about 2 minutes of CPU time before resetting the CPU time
counter to 0.

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
postgres 5682 33.0 0.6 8292 3388 pts/1 R 22:29 0:00 postgres:
postgres TAL2 192.168.255.14 SELECT
[snip]
postgres 5682 40.6 0.6 8292 3408 pts/1 R 22:29 2:02 postgres:
postgres TAL2 192.168.255.14 INSERT

I'm trying to understand why this is.

Which part of the documentation can I read that would help me understand
why it seems one process does something for 2 minutes and then the CPU
time counter is reset to zero?

Jc