It varies depending on the sleep therapy device. For example, on an S9 therapy device a 1GB SD card easily stores 365 days of usage data, 30 days of detailed data, and seven days of high-resolution data. On all AirSense 10 or AirCurve 10 therapy devices, you can get up to 365 days of usage data, and 30 days of both detailed and high-resolution data.