When you're trying to figure out if the millennium in Revelation 20 is a literal thousand years or a symbolic period, it really comes down to your perspective and how you read apocalyptic scripture. Before jumping straight to literalism, we need to compare how numbers function everywhere else in Revelation's genre. In apocalyptic writing, numbers are almost never about strict math. Instead, they carry symbolic meaning, like seven standing for completeness, twelve for covenant order, or 144,000 representing the total body of God’s people. The number 1,000 is no different. It is basically a symbolic way to express a vast, complete era, similar to scripture saying God owns the cattle on a thousand hills.
Whether you take it literally or symbolically usually depends on if you hold to a pre-trib rapture view. Today, the majority of Christians who are waiting for the rapture expect a strictly physical, future 1,000-year reign, but that idea really gained traction in the 1800s when a lot of new, cult-like Christian denominations started popping up. If we look at scripture and history with real discernment, though, things get interesting. There is a strong case to be made that we might actually be living in Satan's little season right now. If that is true, then the millennial reign has already happened. While people like to debate whether the kingdom is strictly physical or strictly spiritual, I believe it is actually a metaphysical reality where both the physical and spiritual meet.