I created a wallet and choose gpg as "best" protection, and use my gpg key. After that, my kwallet is always open without asking me a password or the passphrase of my gpg-key, even at computer starup. I should expect that I should have to give gpg's passphrase at least once after startup or connection, and better, once after sleeping or pause mode (excuse my english) Kde neon 5.12, Plasma 5.12.5, framworks 5.46.0, Qt 5.10, noyau 4.4.0-127-generic
There are options to auto-lock the wallet, but with a GPG wallet, the passphrase is cached in gpg-agent, so it's as good as keeping it unlocked.