Oracle how to manage the flashback log space?
Short answer: Oracle will keep flashback logs as long as it can, limited only by space in FRA.

Longer answer: The database FRA (Fast Recovery Area) is a fixed size, defined by parameter db_recovery_file_dest_size

In this, go various things, including:


Archive logs
Flashback logs
Redo logs
Standby recovery logs
Backups
And more!


Oracle will, under certain circumstances and under space pressure, automatically delete stuff from FRA to make room for new stuff.  It will delete flashback logs at will, even if that brings the possible flashback time under the value that the db_flashback_retention_target parameter is set to.  Remember: It's a target.

If, under space pressure, Oracle can't delete enough stuff (e.g., archive logs that  haven't been backed up) in order to continue, it will "freeze" and give an error on log in.

If there is no space pressure within the FRA, it will continue to store stuff for as long as it can.