As an artist, I work with my art director and game designers to figure out what needs to go into the game. I'll spend part of the day in meetings discussing different possible ideas. Once we've settled on a few, I'll go back to my desk and find research some images online and start sketching some ideas up. Once i've got a few solid ideas, I'll send them over to my art director and the designer to see what works and what doesn't and modify as necessary. Once the sketch is approved I'll start working up the colors and actually paint up a finished version.
The sketching and painting process can go quickly or take a while if we have different ideas and need to go through many variations. When working on new games and creating new art styles, the process of figuring out the "look" of the game will take several weeks to months depending on the complexity of the visuals and the needs of the game.
This entire process is fun for me. But we do take breaks and blow off steam, grab food, and just relax and play games. At Zynga, the culture is such that we can budget our own time as long as we get our work done and hit our deadlines. For me, that will occasionally include taking a break to play video games or ping pong with my co-workers. If we take longer breaks, then we just work later hours. So yes, we do work hard, but we also still have fun. If we don't have fun playing and making games, it's hard to get other people to enjoy our games as well.