the different stages of mitosis take different lengths of time to complete.
at any given time, each cell will be at a different point in the cell cycle.
so you can expect that the percentage of cells observed in a stage of mitosis will be about the same as the percentage of the length of the cell cycle that that stage lasts.
so you calculate the % of cells in anaphase (as a decimal between 0 and 1), and multiply this by the total length of the cell cycle (in minutes)