Certain people who argue against the existence of an eternal afterlife like to argue that it would feel like “hell” or that we cannot even imagine it. I disagree. There are ways that we can guess what it would be like, and to find a clue to this question, all one needs is a little bit of calculus.
While it might be hard to imagine certain aspects of an afterlife, such as what it would literally feel like, if we do not have bodies, the eternity question is fairly easy to address. Consider our own lifetime to start. As a child, summer vacation used to last forever. Or at least it felt like it did to me. By the time high school rolled around, it did not feel anywhere near as long. As we get older, it seems that the days, weeks, months, and even years just fly by, and on longer timescales, this sense feeling has been identified as being fairly common as people age (Scientific American). This change in view makes sense as we experience longer periods of time. If we can extend this feeling to eternity, we can actually estimate how long eternity would seem.
Of course, the actual result depends on a few assumptions. But consider our one lifetime as a start. Suppose each equivalent amount of time from then on feels like 90% of the last amount. As an example, suppose we lived to 100 and the first 100 years of our afterlife felt like 90 years, our second 100 felt like 81 years, and so on. Even though we would still be living forever, the sum 100 + 90 + 81 + … + 0.9^n * 100 + … has a finite sum. This type of series is called a geometric series, and the infinite sum turns out to be 1000, or 10 lifetimes. The exact length depends on the fraction by which each unit of time feels shortened.
There are other progressions which are also finite, and there progressions which do not converge to a finite sum. But if we can extend our experiences in life to our experience in an afterlife, it does seem like eternity would not feel like it would last forever. Therefore any argument that rejects an eternal afterlife based on the notion of a torturous never ending existence can be seen as being too limited to hold water. One would first have to show that our experience in the afterlife is not modeled by this diminishing perceived-time phenomenon.