What is the life span of a gearbox?

Could you please elaborate on the life span of a gearbox? I understand that it depends on various factors such as the quality of the materials used, the operating conditions, and the maintenance practices. However, I'm curious to know if there's a general estimate or a range that one could expect for the lifespan of a gearbox under normal circumstances. Additionally, what are some of the key factors that contribute to the premature failure of a gearbox, and how can we prevent such failures from occurring?