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Yes, you can wait 30 days between downloads and not lose any of the detailed graphs. Maybe even longer.
With the prior ResMed S9 models it was 7 days. On the eight day the S9 would start overwriting the files that contained the detailed graphs. Oldest got overwritten.

I think with the AirSense you can go longer than 30 days and not lose details but I haven't personally verified. Just going by what others have reported.
I don't think I yet have 30 days on my AirCurve to verify..but I will check.

I don't know just how long for Respironics machines...I am pretty sure it is much longer than 30 days for the detailed graphs. The Respironics machines actually hold everything but the flow rate graph in its internal memory so Respironics users don't lose all the detailed graphs if the SD card is out of the machine during the night. Respironics won't start overwriting files until the SD card gets full and that takes a lot of files. Most cards will hold years of data before getting full.
That's why I was always unhappy about the S9 overwriting files...the card could hold a lot more and there was no need to overwrite.

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One of the things I did early in my therapy when I was watching the data closely was that I elected to download in the evening instead of first thing in the morning. That way I wouldn't have a preconceived notion as to how I slept or didn't making a self fulfilling prophecy. If the numbers were bad I would think "oh crap, today is going to be bad" and it would be but maybe I created it in my head.

I have this story to tell. About 3 months into therapy (late Aug of 2009) I had a day where I felt amazing. Up to that point I really hadn't been feeling all that amazing except for the nocturia and killer morning headaches going away. Those 2 symptoms going away were really the only way that I had to validate that the therapy was working. I sure didn't feel like superwoman otherwise. Anyhow, I woke up one day and really felt "good"...got lots of stuff done that day and actually felt amazing. I thought OMG this is what it is like...I finally turned the corner and had the "miracle".
At any rate I didn't look at the data until right before bed. I just knew that I must have had an AHI the night before of 0.0 or something ridiculously good.
So I get ready to really pat myself on the back and much to my surprise the AHI was 10.2...yep...10.2 and it was an UGLY report but I felt amazing that day. Still can't explain feeling great with a crappy report except that stuff happens we can't explain.
I am pretty sure if I had seen that report early in the morning that I wouldn't have felt amazing during the day.

Oh...that miracle day....I still don't have them very often but occasionally I do. I know that it's more related to hours of sleep and the quality of those hours than it is what is shown on the reports.
I went all of 2010 and only looked at the software data once and that was because I changed masks and wanted to see the leak rate.
Even now I don't download daily unless I am experimenting with something. I got a new AirCurve not long ago and set my S9 aside for the time being.
So I am watching the data a little closer but with the S9...I would go months between downloads and not really care that I was missing some detailed graphs.

Initially looking at the data was really important to me and I understand the need to but after a while it gets boring because it doesn't change that much and since I never could really spot a correlation between data shown and how I felt I got to where I wouldn't have the need to look all that often.
The data shown didn't really tell me anything I didn't already know...that how many hours I slept and the quality of those hours were the real driving force in how I felt during the day. For me those hours of sleep and the quality were affected mainly by stuff unrelated to sleep apnea. The machine was doing its job in terms of sleep apnea prevention but it couldn't prevent stuff from happening unrelated to sleep apnea that would sometimes mess with my sleep. The machine can't fix problems unrelated to sleep apnea and I just have some problems it can't fix.

I did learn that getting an extra 45 minutes to an hour of sleep made a huge difference in how I felt...and that holds true to this day.
I can get the prettiest software report in the world with no leaks and essentially 0 AHI and still feel like total crap if I don't get enough hours of good quality sleep.