Note: I wouldn't put things the same way if I were to write this again, but I leave it here for interest. Despite hesitations about the way I framed this I still think the point I was making has some validity.
People sometimes suggest that the Hebrew and Greek words translated as "eternity" or "forever" in the Bible have been misunderstood, and could simply mean "a long time". This suggestion is often made to cast doubt on the idea of heaven or hell lasting forever.
A recent twist is to emphasise that Jesus was probably speaking Aramaic, so the Greek αἰώνιος (aionios) in the New Testament should be understood in terms of the underlying Hebrew עולם (olam).
This argument can sound compelling, partly because there is an element of truth in it, but also because it is an appeal to authority. Most Christians do not speak Greek and Hebrew, and so it seems difficult to object to what seems like superior knowledge. Most of us love the idea of discovering secrets missed by others, so when a pastor tells you that all the translators have made a mistake, and the truth can now finally be revealed, it sounds fascinating!
It is true that sometimes עולם can mean "a long time". But it doesn't always mean "a long time"—and more importantly, it normally means "forever".
By way of analogy, the English words eternity and forever can just mean "a long time" too (as in, "this bus is taking forever"). To then suggest that they never mean anything other than "a long time", or that the basic meaning is not actually forever would be ridiculous! Both English words have a semantic range, a range of possible meanings, and we have no difficulty distinguishing what the intended meaning is when the word is used. When talking about a bus taking forever, we are clearly using hyperbole or exaggeration to make a point. Similarly, most of the time it is not difficult to figure out how the Hebrew (or Greek) word is being used, and which of the range of possible meanings is intended.
Sometimes עולם does just mean a long period of time. Take 1 Samuel 27.12, where Achish thinks that David will serve him "forever". Achish is not delusional: he knows full well that both he and David are mortals, and so he is using hyperbole, using exaggeration. It is not hard to work out.
Sometimes it means the distant past. Take Malachi 3.4, for example, where you have the "the days of olam"—the "days of old" or "days gone by". It is not hard to work out.
But the basic, normal meaning is just eternity—"usually eternity", says HALOT (the main Hebrew and Aramaic lexicon). So, we have God's goodness or kindness (חסד) often described as being forever (עולם), for example in Psalm 107.1. Nobody thinks that the Psalmist means, "God's love lasts as long as a boring bus ride", or, "God's love endures for really quite a while".
God himself is described as being from eternity to eternity, for example in Psalm 90.2. There, God's eternal existence is contrasted with the fleeting lives of mortal men and women. The whole point of the Psalm is that God is eternal, in the normal, simple meaning of the word. He had no beginning, and has no end. Nobody thinks that the Psalmist means, "you've been God for quite a few years now", or, "you are going to carry on being God for a fair bit longer—but not forever".
In a passage, if someone wants to argue that olam doesn't mean forever, they need to demonstrate why. The most common meaning is just forever, and they need to show from the context why a different meaning is intended.
So if you read or hear someone claiming that olam always (or normally) means a long time, then they are not giving you good information. They are telling you something partly true in a misleading way.
Think about it this way: Bible translators are experts, and they do a brilliant job. If they think that the best English word for olam is "forever" or "eternity", they will normally know better than an individual pastor.
And that works as a good general principle too. For words of such theological importance, it is very unlikely that an entire committee of translators would have made such a basic error. If you wonder whether one particular translation has got it wrong, try a couple of others and see what they say. And then decide who you want to trust: the experts on the translation committees, or one person claiming that the rest of the church has been confused for centuries.