What is a safe amount of characters in a passphrase?

The coldcard and trezor both has support for a passphrase. What amount of characters in this passphrase that would be considered unbreakable with current technology and within some years of bruteforcing?

Depends. Best to use a passphrase pulled from wordlists, like EFF Diceware's or BIP39, so then six words.

But do not go over 49 characters.

You can get away with a string under 20 characters if the characters are truly random generated, upper/lower.

Using a password manager is a great way to go if you don't want to use physical dice or coins.

Whatever you do, do NOT create one from your own brain, easily crackable in a short time no matter what, must come from high entropy, randomly generated.