Hello,
there are a couple of problems with your calculations, first, there is a bonus at the end of each round depending on the number of pieces of Atlantis that survives the assault. Its fairly easy to get to 50,000 without losing any pieces of Atlantis resulting in a large bonus, as you continue to play into the higher levels, you generally have fewer pieces left to receive your bonus. this results in it taking longer to build up your score.
Try letting most of Atlantis get destroyed, and play for 5 minutes, you will notice your score is no where near 50,000. There is also another reason why it takes so long to get up that high... but if I go into details it would give away the secret....
Remember Numbers are a funny thing, ask someone what the odds are of winning the lottery, most may say 1,000,000:1, someone else may say its 50/50 either you win or you lose....
Then again, I am going from memory about a game I played 28 years ago… I believe I started playing before 8 am, and ending sometime after 10pm… Then getting my father to take a picture with the Instant camera.