After 30 minutes a mask starts to lose its efficacy. After 2 hours, you might as well not be wearing it at all because of moisture from your breath make the mask wet. This assumes that the wearer has not touched his/her face the duration. Once you touch your face to adjust the mask then it's game over. It's all in the hygeine of the hands people. Keep em clean! And for god's sake keep your filthy hands off your face.