According to this Campsonic exposé of the current issue of GamesTM, Sonic Unleashed lead designer Yoshihisa Hashimoto says that each level will last around five minutes, so that the player never becomes bored.
“There are a couple of guidelines we followed in designing the levels,” he said. “The first is that each 2D or 3D section should last around 15-30 seconds. The second is that the overall stage should be no more than five minutes long.
“We try to switch between 2D and 3D as often as possible so that no one sequence lasts too long. The idea is to create a pace that never allows the player to grow bored.”
More through the link.
By Mike Bowden