How do you know when to understand the Hebrew word עלום (olam) as "eternal" or simply a long duration?

Question

In Gen. 9:16, עלום is translated as "everlasting" in reference to the covenant of the rainbow:

וְהָיְתָה הַקֶּשֶׁת בֶּעָנָן וּרְאִיתִיהָ לִזְכֹּר בְּרִית עוֹלָם בֵּין אֱלֹהִים וּבֵין כָּל נֶפֶשׁ חַיָּה בְּכָל בָּשָׂר אֲשֶׁר עַל הָאָרֶץ

And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the earth. (KJV)

In Dan. 12:2, עלום is translated as "eternal" ("everlasting") in reference to the life received by those who awaken from their sleep (i.e., in the resurrection):

וְרַבִּים מִיְּשֵׁנֵי אַדְמַת עָפָר יָקִיצוּ אֵלֶּה לְחַיֵּי עוֹלָם וְאֵלֶּה לַחֲרָפוֹת לְדִרְאוֹן עוֹלָם

And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt. (KJV)

However, in Psa. 143:3, should it be understood as "eternity" or simply a long duration?

כִּי רָדַף אוֹיֵב נַפְשִׁי דִּכָּא לָאָרֶץ חַיָּתִי הוֹשִׁבַנִי בְמַחֲשַׁכִּים כְּמֵתֵי עוֹלָם

For the enemy hath persecuted my soul; he hath smitten my life down to the ground; he hath made me to dwell in darkness, as those that have been long dead. (KJV)

Answer 1

As a matter of semantics, the Hebrew word עלם ('olam) means something more like the English words 'age' or 'era', in the sense of a distant time period.

עלם is occasionally translated as 'everlasting' or 'eternal', with a meaning of 'age-enduring'. For example, some passages describe hills or mountains as עלם (Genesis 49.26; Deuteronomy 33.15; Habakkuk 3.6), because from the view of the writers, mountains endure countless ages; from human perspective, they are 'eternal'.

So as @H3br3wHamm3r81 said above, how עלם should be translated is entirely dependent on context. Relatively speaking, few words have an absolute one-to-one translation in any given language.