At which frequency run CTXSYS.CTX_DDL.OPTIMIZE_INDEX(...) on a fulltext index? I've been told to run periodically CTXSYS.CTX_DDL.OPTIMIZE_INDEX(...) on some fulltext indexes, with no more information, in particular at which frequency this task should be run.

I guess that it depends on the amount of indexed data, the time after which the index does not "performs well" (maybe after a lot of writes), the time taken by running  CTXSYS.CTX_DDL.OPTIMIZE_INDEX(...) (is it "blocking" DB when running?), etc. so the answer to this question may not be very precise.

However, I would like to know if there are some general rules, principles, recommendations to follow (couldn't find any).
You can try to use the index statistics report by first enabling reporting on the index  (BEGIN ctx_output.enable_query_stats(ix_my_indexname); END;).  Then you can use the procedure ctx_report.index_stats.  While sifting through the CLOB of info in the report, the most useful information to me is the FRAGMENTATION STATISTICS...estimated row fragmentation: 23%

see Oracle documentation here for more thorough examples of the stats report.