How long does a star stay in a nebula?

A star does not "stay" in a nebula; rather, it forms within a nebula. A nebula is a vast cloud of gas and dust where star formation occurs, and the process can take millions of years. Once a star forms, it will eventually evolve and leave the nebula, transitioning into different stages of its lifecycle, such as a main-sequence star, red giant, or supernova, depending on its mass. Thus, a star is only associated with a nebula during the initial stages of its formation.