In a sample, the growth of microbial cells started with an initial concentration of \(5 \times 10^4\) cells per millilitre (mL) of the sample. After a certain time period, the cell concentration was found to be \(1280 \times 10^4\) cells per mL of the sample. Assuming binary fission of cells and no cell death, the number of generations of cell growth occurred in this time period is _______ (answer in integer).