According to those that believe hell will be "eternal conscious torment" or "eternal separation", if the flames and worms are not literal, then why is it necessary that the word "eternal" be a literal duration? In other words, why must "eternal" be a quantity rather than a quality?
Biblical prophecies use symbols and analogies to convey truth. Very rare is the prophecy that is entirely literal. However, every symbol represents a concrete reality.
One commentator on prophecies (Ellis Skolfield) studied the context and the rest of scripture to understand the range of meanings that a time related word can hold. There are places where a day means a day, but in another place, it can mean a year. A week can mean seven days or seven years. Likewise a time can be a thousand years and a season can be one quarter of that, or 250 years. A month, being thirty days, can then mean thirty years, and an hour, being 1/24th of a day, is two weeks or so. But in every case, the prophecy takes a time period and applies a multiplier to it.
If you take the words eternity, forever, eternal, you are applying an infinite multiplier. No matter what you start with: minute, hour, day, week, year, century, millennium - if you multiply by an infinite value, you get an infinite time. BY this reasoning, eternity is not an analogy or a figure of speech or a symbol, it is literal.
There is another possibility. The King James renders one verse differently from modern translations. Revelation 10:6 says:
6 And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer:
Some people hold that this means time will actually stop. Time as a feature of reality will cease to exist. In this case, we enter eternity and an unchanging state. In that case, eternity means existence in an unchanging and unalterable state. In that case, Hell means existence in a timeless, unalterable state of separation from God.