Well my point with 36 pixels is that it would let people use the site in a healthy way (daily) but also allow decent velocity of creation for a slow-developing canvas. The details could be tweaked I just figured "once a day sounds right, no pressure then" and "150 users doing 24 pixels per day would fill a 10th of the canvas (the scroll-right number, 100,000 pixels) in a month. Obviously the numbers could be tweaked depending on activity of the community and the dimensions of the editable canvas, I would just say targeting a good experience for "daily use" is healthy for a permanent site. Because I know my use of this site this weekend was anything but healthy.

Maybe put the cap at 16 (assuming hourly pixel generation), so you can hit the site twice daily plus a little wiggle room while still reaching peak output. Too often the canvas felt like being the little Dutch boy with the finger in the dike.

Imho the approach of punishing overwriting with slower regen is the right move - every overwrite pixel you place should add a 1 pixel delay to your pixel generation, so vandalizing (and repairing) takes twice as long as creating.