In the games industry, it's usually the project manager who creates and enforces production schedules. To do that, a list of task and assets to produce is assembled based on the design documentation. These tasks are then paired with estimates that (in the best-case scenario) come from the people who will actually end up doing said tasks. If the project is highly inventive and/or staff members inexperienced, more senior employees might get involved in helping with the initial schedule.
Unfortunately, in the words of strategist Helmuth von Moltke, "No battle plan ever survives contact with the enemy." First production schedules are likely to be very, very wrong. In the beginning, things will usually take longer than anticipated, and a lot of unknown problems, changes in direction, and missed tasks will emerge. Fortunately, as time goes on, your team's experience with the product grows and your tools are likely to improve, making the creation of future game content much...