The CAPM is based on the assumption that all investors have identical time horizon. The core of this assumption is that investors buy all the assets in their portfolios at one point of time and sell them at some undefined but common point in future.
What are the assumptions of the CAPM?
The CAPM has serious limitations in real world, as most of the assumptions, are unrealistic. Many investors do not diversify in a planned manner. Besides, Beta coefficient is unstable, varying from period to period depending upon the method of compilation. They may not be reflective of the true risk involved.