Answering for myself now:
Well, because "as a thousand years is like a day to God" is a thousand years for us humans and a day for God, so that is consistent with what the Rev 20 text says.
Applying 2 Peter 3:8 to John 2:19 in which "Jesus answered them, 'Destroy this temple, and in three days I will raise it up" would imply 3 thousand years from His death he'll return. He was crucified around 30 AD, so some (not I) expect His return in 2030 AD when the start of the 'third day' begins (and they compute the tribulation to begin in 2023) and 1000 human years later it would end. This application of 2 Pet 3:8 argues for a literal 1000 years, but also implies we can compute and know the date of Christ's return, which only the Father knows, so I discount this viewpoint.
Could it be longer than 1000 years? I suppose, but again other durations of time were literal: 40 days and nights, 70 sevens, etc. I've no good reason to think it longer than 1000 years, and one (or three) human day doesn't work, so 1000 years seems most consistent with all scripture until I know better.
I don't single out Rev 20 for literal interpretation. I try (albeit not always well) to interpret all of Revelation and all scripture literally, unless scripture informs me otherwise (because it is a parable for example).