Modeling DB table. Is the account id (public key) string length always 56 and always will be?
It's always 56 for the current encoding scheme. And there is no intention to change the keypair generation mechanism in the nearest future, as far as I know.
The invention of the relatively cheap quantum computers with more than 100 qubits may lead to cryptography schemes reconsideration. But they won't be available for at least 2-3 years.
Therefore, you can safely assume that all Stellar addresses will be 56 chars long.