S9 files on memo card, which contains what, how often backup

[locomotive_breath]

Half of the files are just CRC checksums.
The rest is half from the beginning and half from the ending of the sleeping session.
These EDF files are overwritten monthly (EVE, PLD & SAD) respectively weekly (BRP).

Now the questions: What do the BRPs contain?
Have I to perform weekly backups in order to save "all data" in the long run? Or is monthly enough?
Any way to prolong the 'wrap-around' time?