Methods and compositions for determining the duration of a cell cycle phase of a mammalian cell, as well as identification of the cell cycle stage of fixed mammalian cells, are provided. In practicing the subject methods, at least one biosensor polypeptide that monitors a cell-cycle phase in a mammalian cell, such as mitosis, G1, S, or G2 phase, is used to determine the duration of a cell-cycle phase of a mammalian cell. Also provided are methods for identifying an agent (e.g., a gene product or small molecule compound) that modulates the duration of a cell-cycle phase of a mammalian cell, as well as kits and systems for practicing the subject methods.