You need private keys to access your coins. So, basically you need just a backup of your private keys.
The wallet.dat file contains your master private key (which is needed to derive all further private keys).
Therefore it is more convenient to simply backup the wallet.dat instead of backing up each private key individually.
And in how many days I should take backup.
You only need to backup once (assuming you don't change your password). If you change your wallet password, a new backup is required.
You don't need to back it up each X days. Just after each password change.
Also make sure to have multiple copies spread. And do NOT keep all backups connected to a power source (one lightning strike could easily roast all of your hardware connected).
Ideally, you make an paper-backup of your master private key (starting with
xprv..). Just to be on the safe side.
And how I can make recovery of my balance in both cases if my node goes down.
If your node simply crashes, your wallet.dat file shouldn't be corrupted at all.
Simply start core and let it repair the database itself (it will redownload, resync, whatever is necessary..). But your private keys stay untouched.
However, if somehow the wallet.dat gets corrupted, you simply need to install core (assuming its not installed) and place your wallet.dat into the data directory.
Then startup core. That's it.
