diu

Diu answers "how long", not "when". That distinction does a lot of work: diu vixit is he lived a long time, olim vixit is he lived once upon a time, and iam is "already". The comparative is diutius and the superlative diutissime, both irregular and both common, and diutius with a negative is the ordinary way to say "no longer".

One construction to watch. Iam diu, iam dudum or iam pridem with a present tense describes something still going on: "iam diu te exspecto" is "I have been waiting for you a long time", not "I waited". English switches to a perfect there and Latin does not. With a past tense the same phrases mean the action had already been running a while.