Ah, okay. I researched until the command line tools.
While https://github.com/input-output-hk/cardano-node/blob/master/doc/reference/building-and-signing-tx.md makes it sound like you have to set --invalid-hereafter to something (then there would not have to be a default, because every transaction would have to choose), the examples in https://developers.cardano.org/docs/native-tokens/minting/ do not set one at all, while the examples in https://developers.cardano.org/docs/native-tokens/minting-nfts/ do (for NFTs it has to be set, because it needs to match the minting policy). Note that examples with a TTL years in the future seem to work.
To know, what exactly happens, when nothing is set (and if there perhaps is a maximum in addition to the set invalid-hereafter parameter), you might need to dust off your Haskell skills. 