sooner than or sooner then?

Sooner Than Later

Than is used in comparisons as a conjunction (as in “she is younger than I am”) and as a preposition (“he is taller than me”). Then indicates time. It is used as an adverb (“I lived in Idaho then”), noun (“we’ll have to wait until then”), and adjective (“the then-governor”).

Bob Dylan – One of Us Must Know (Sooner or Later) (Official Audio)