The development of the general theory of relativity by Albert Einstein was a complex and lengthy process that spanned several years. The first seeds of the theory were sown in 1907 when Einstein began exploring the equivalence principle, which states that gravitational and inertial mass are equivalent.
Einstein's work on the theory progressed through a series of steps and refinements. In 1912, he formulated the equivalence principle more precisely, realizing that it led to the conclusion that gravity is not a force but rather a curvature of spacetime caused by mass and energy.
The final form of the general theory of relativity was published by Einstein in 1915. However, it is important to note that the process of deriving and refining the theory involved numerous challenges, mathematical complexities, and interactions with other physicists. Einstein collaborated and corresponded with various mathematicians, including Marcel Grossmann, who provided crucial mathematical insights and assistance.
The development of the theory was influenced by Einstein's intuition, physical insights, and thought experiments, as well as by mathematical calculations and discussions with fellow scientists. It took years of careful and persistent work to arrive at the complete framework of the general theory of relativity.
Therefore, while it is difficult to assign an exact timeframe, we can say that the process of developing the general theory of relativity took several years, from the initial formulation of the equivalence principle in 1907 to the publication of the final theory in 1915.