genre
zero-player simulation.

average playtime
Mathematically speaking; until the sun burns out.

Describe the difficulty of the game
It's a zero-player game, thus there is no difficulty. It's not all that easy for your Computer tho, and that's why there are performance spikes, every now and then.

About your game
If you're familiar with Conway's Game of life - it's exactly that, on a 3D grid, with some additions to the ruleset.
If you're not familiar with it - go look it up, it's magnificent and should be common knowledge.