すでに・とっくに: the difference
すでにN3
Meaning
Already, Too late
Past papers
Example
時既に遅し。
You are already too late.
とっくにN2
Meaning
Already, A long time ago, Long ago
Past papers
Example
「バス？とっくに行っちゃったよ。」
'The bus? It has already left.'
How to tell them apart
「もう」: the plain one; just says it is already done. 「もう食べた」 (I've already eaten).
「すでに」: formal and written; it is already finished at the time of speaking. 「すでに提出済みだ」 (Already submitted).
「とっくに」: spoken and a bit exaggerated: done a long, long time ago.