Each tile they post takes a point.

Points regenerate at a rate of 1 every 12 hours (second by second in fractional points).

If you have less than 2 points, and another artist posts a tile, the rate doubles. And doubles thereafter for every artist who posts while you have less than 2 points (this could be tweaked a little).

Thus an active grid supports many active artists. But one artist will find it difficult to dominate the grid alone in any reasonable amount of time.