I've never managed to stay in a gym longer than a few hours, BUT if I did I was just wondering if the 500 + 10 bonus increases with each 21 hour cycle?
As of the new update that was just released, the length of time your Pokémon defends a gym determines the defender bonus entirely.
First of all, the defender bonus is now based on the length that your Pokemon stays in the gym, and so the longer that it remains defending a gym, the higher your eventual defense bonus will be; you won’t just receive a set bonus once a day.
But you also do not collect the bonus once every 24 hours anymore. Instead, you only collect it when your Pokemon is no longer defending that gym. Whenever the Pokemon returns to you from the gym, it will bring with it all of the coins that you have collected over that entire period. You still can only collect 100 coins per day maximum, though. You will receive a notification when the Pokemon returns home from the gym informing you how many coins you have collected.