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).