The sensors attached to each logger vary, but generally I am juggling 96 ascii characters per record. I am using pString, even for number data, because it gives me protection from buffer overflows, which is REALLY important for a coding newbie like me who is swapping sensors all the time. When I concatenated the recent test data into a single csv file, it came to around about 40 mB in size. (about equivalent to 1 year at a 1.5 minute sample interval)
Also, I have been doing tests with the Rocket Scream Mini Ultra boards, and they are delivering incredibly low sleep currents, right out of the box. I am sure they ship these things with the BOD fuse turned off, which I am not that happy about for a logger application, but still, they are delivering the same kind of performance I have been seeing from the unregulated tiny-duino boards. I have test's ongoing to see how gracefully they handle the situation when the power supply voltage falls below the rated input on their voltage regulator. The MC5205 regulators on the regular Pro Mini & clone boards go into a 1/2 second "Start & Shut down again" loop when this happens, which I am sure is not good for the SD cards?
What I would like on systems with voltage regulators, is a more graceful fail over that simply passes the power supply voltage through when it falls below the regulators spec'd input. Some of my sensors would would start to deliver bad data at that point, but some are much hardier, and would keep going fine until I reached the SD card cutoff around 2.7 volts.