From Scripture, how long is a day?
It depends on the context, doesn't it? In Genesis, God created the world in six "days" and specifically said "evening and morning" were what counted as a day. To humans, in THIS earthly life, a day is approx. 24 hours. But with God - because He is eternal - time is not binding on Him which means our thousand years could be as one day with Him. Not everything is a pat yes or no answer, is it?
Now, can you explain why two Roman Catholics believe two different things about the "good" thief's eternity?