längst
long ago / already
Expresses that something has already been the case for a long time or happened a long time ago.
Als ich ankam, waren die anderen längst gegangen.
When I arrived, the others had long since left.

long ago / by now
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long ago / already
Expresses that something has already been the case for a long time or happened a long time ago.
Als ich ankam, waren die anderen längst gegangen.
When I arrived, the others had long since left.

'Längst' is a form of the superlative of 'lang' (long). It expresses that something has been this way, or was this way, for a very long time already.
Diese Technologie ist längst veraltet.
This technology has long been obsolete.
The meaning of 'längst' changes completely with 'nicht.' On its own, 'längst' means 'already for a long time,' but in the combination 'längst nicht,' it means 'nowhere near' or 'by no means.'
Er ist längst fertig mit seiner Arbeit.
He has long since finished his work.
'Längst' often has a subjective nuance and expresses surprise, impatience, or reproach. It emphasizes that something happened much earlier or should have happened much earlier.
Du kommst jetzt erst? Ich warte schon längst auf dich!
You're only coming now? I've been waiting for you for ages!
The combination 'längst nicht' is a fixed expression that strongly weakens or negates a statement. It is often used to emphasize that a state has not yet been reached.
Obwohl wir schon drei Stunden arbeiten, sind wir längst nicht fertig.
Although we've already been working for three hours, we are nowhere near finished.
already
'schon' and 'längst' can both say that something is not only happening now. 'schon' simply means 'already' or 'earlier than expected'. 'längst' is stronger: it says that something has already been the case for a longer time or happened a long time ago. Learners confuse the words because both have to do with time. If what matters is only that something has already happened, 'schon' fits. If you want to emphasize that something is actually long known or long since done, 'längst' fits better.

Herr Neumann kennt die Nachricht längst, weil ihm ein Freund gestern davon erzählt hat.
Mr. Neumann has known the news for a long time, because a friend told him about it yesterday.
Sie ist schon zu Hause, obwohl der Kurs erst um fünf endet.
She is already at home, even though the course does not end until five.