I recently came back from Friends for Life (conference for t1 diabetics), where I attended a session on CGMs by Gary Scheiner (author of Think Like a Pancreas). He suggested that as long as the cgm is accurate and the site looks fine (no inflammation, etc), you should be able to wear you cgm long past it "change the sensor" alert (all the way up to 30 days).
I have previously been able to wear my medtronic cgm for 7 days and then had to remove it to recharge it. Gary suggested that once you recharge it, you should be able to reconnect to the old sensor and continue wearing it. So far I have not been successful at this. After recharging and reconnecting the cgm to the old sensor, it lasts another day or two and then starts giving me crazy readings or gives the "failed sensor, replace sensor" message. I was wondering if anyone else had experience with making the medtronic cgms last longer than 7 days.
I don't know if it's true, but I heard somewhere that the sensors have a built-in "function" to harass you with Weak Signal messages if you extend them beyond the three-day mark. I've noticed that, at times, I'll get Weak Signals for just enough time to require a calibration and 2-hour warmup, but then it works perfectly again.
Also, a "Weak signal" could have to do with the actual insertion site not getting the electrochemical reaction from the interstitial glucose. My sensors generally last about 6 days - maybe 7, but not more. There's a gel on the filament that goes beneath the skin, and that gel reacts with glucose to create the electrical impulse that is measured. The higher your glucose level is, the quicker the gel depletes and the shorter the sensor lasts (or so I'm led to believe. I can't be absolutely sure). To last 30 days, I believe your sugars have to be darn near perfect all of the time.
I usually get 7-8 days per charge, but the one or two times that I have reattached it, I have never gotten more than 2 days before readings go wild. Not with the effort to me then, so I just replace the sensor with every recharge.
