How often should one backup their wallet? Is it enough to do it once after encrypting with new password?
For the full node wallet, you need to back it up after encrypting it and again after adding any new addresses. It is not necessary to back it up after every transaction, just whenever you generate a new address is enough.
For the mobile and thin clients, you should be able to retrieve all balances by using your 12 word seed phrase, so there is no need to back anything up apart from the phrase.
Cheers,
Craig.