Thanks Howard, I needed your help to find the right page among 247 administrator links.
I set 'Keep logs for' it from 'Never delete logs' to '365 days'.
moodle.mdl_logstore_standard_log
table now is 42.1 MiB for 413541 rows. My database now is 126 Mb, gaining about 260 Mb. My backup is very happy.