2-3 generations. According to China Education and Research Network official website, the recovered cells have to be passaged for 2-3 generations before they can be used in experiments. Cell resuscitation refers to thawing cells frozen in liquid nitrogen or -80℃ refrigerator, and then culturing and subculturing. Cell resuscitation should adopt the method of rapid melting, so that the extracellular crystals can be melted in a short time, and the slow melting can be avoided, so that water can penetrate into the cells and recrystallize, which will damage the cells again, thus affecting the survival rate of cells.