とっくに is used to say that something was already done or happened a long time ago, often implying that the speaker thinks it should have been done much earlier or that it is old news. It carries a nuance of 'way back' or 'ages ago,' and it often appears with past-tense verbs, but can also be used with present state verbs to show that the state has been true for a long time.

Unlike もう (which just means 'already'), とっくに stresses that the time gap is large. For example, if someone says 'もう終わった' they just mean it's finished, but 'とっくに終わった' means it finished a long time ago, so why are you asking now? This makes とっくに handy for expressing mild surprise or impatience.

It is a common adverb in both speaking and writing, but it is not overly casual—you can use it in everyday conversations without sounding stiff.