I have managed to get 21 -28 days out of every single G4 sensor that I have used.
Typically I apply it normally without any extra tapes or glues, after day 5ish i usually notice the edges starting to lift at which i cover it with a piece of clear medical adhesive with a hole cut in the middle for the transmitter and then just change that whenever it begins to peel making sure that when i remove it i don't peel off the original adhesive pad. This system has yet to fail on me.
Every one of my sensors have been placed somewhere on my love handles/ rear abdomen, so far no issues!
EDIT - It is worth mentioning that day 1-3 are always fairly inaccurate every time you start / re-start a sensor, and around day 10 i notice that the highs and lows get 'blunted' and it reacts slower to fluctuations, at around day 15 i start to see the '???' at least once a day for a couple hours lol
Typically my levels are fairly flat so these issues never bothered me. If i was someone who has frequent highs and lows, or needs constant CGM then I would change my sensor somewhere between day 7-10. They become more and more unreliable as they pass day 7.