yes "always backup your pass phrase and private keys" actually tells nothing about the intervals, making this a lucky game when using this application over a long time (over months/years)
It feels unclear when to actually make a backup.
Did you read my post? (actually, multiple users said the same above).
You only need to backup your seed once.If you want to be "safer", you can backup each one of your private keys after they are generated. But that not necessary as the seed will do the work for you and restore all your addresses when you try to restore the wallet.
I try to nail it down:
When adding new private keys or new currency or making over 100 transactions i must refresh my backup.
No... You only had to do that in Bitcoin Core's old version, where (IIRC) you had to backup your file every 100 new address.