I have been using the Airsense for just under eight months and the SD card is not quite half full
899MB used, 989MB free, out of the 2gig SD card. Based on this usage the 2gig ResMed SD card should easily hold 1.5 years worth of detailed and high resolution flow data. The data is not being overwritten at the present time and I suspect it won't begin to overwrite old data until the SD card is full, just as the clinical manual indicates.
About a month ago just to satisfy my curiosity I created a new user profile in Sleepyhead and did a data import to that new profile from my SD card. After the data import I looked at statistics page, and daily detail page and all of the detailed data and associated graphs were there in their full glory. So my little Sleepyhead experiment confirmed that a full seven months of data (at that time) was all there on the SD card, nothing was overwritten, and there was no 30 day limit on the data.