If you say that something will happen sooner or later, you mean that it will happen at some time in the future, even though it might take a long time.

What does sooner or later mean?

: at some uncertain future time : sometime.

What is a suitable adverb for sooner or later?

Adverb. sooner or later (not comparable) (set phrase) Eventually, at some undetermined time in the not-too-distant future. He'll turn up sooner or later.