This is a great group to find a lot of useful information. I had gone through about 20 sensors before I knew I could just restart them (can't believe I didn't think of it). Before I learned to restart them, I only had about 2 of the 20 that didn't make it to the 7 days. If that ever happens just call Dexcom and they will replace it. Make sure to keep the wrapper it comes in because they will need the lot number and everything.
As far as going past the 7 days, I've tried it on three sensors now. The longest I have gotten is 14, most recently 9. There is a conspiracy that Dexcom has gotten better about manufacturing them so they don't last for 20+ days like they used to...and from what I've gathered, it is likely true.
The best advice I could get you is in how to make the adhesive last longer than the sensor. This is my routine:
1)Shave the site (I prefer belly but that's a whole other discussion) the night before you are going to insert it (I give it 12 hours prior because shaving leaves moisturizer oil on your skin).
2)Shower the next morning, scrubing the site with soap, and then an oil-free fragrance free face wash (I use that orange Liquid Neutrogena stuff).
3)Dry completely, then scrub with alcohol swab (or clorocept prep stick), and I mean scrub the area, not just wipe.
4)Carefully adhere to skin avoiding any "wrinkling" of the adhesive.
5)Insert...I've found the faster and more fluid I get the plunger in, the more accurate the sensor is. I get intimidated by it every time even though I've done it plenty of times now, so there have been times I kind of got it in 80% of the way then had finish it off. It could have just been variation in sensor quality, but I still think it's more accurate with one swift plunge.
So after this, the edge will start to show slight peeling at day 6-8 (usually at the top). At this point, I add some Flexifix around the edge, not over the sensor (~$20, roll will last you ~1year), and this thing isn't coming off before sensor goes crazy and dies.
Once you get the ??? on day 9-15, I try to recalibrate after an hour or so, but if it still gives you more ??? or really jerky readings, I know it's time for a new one.
It is definitely different for everyone, but this is what I've learned in the last year, so hopefully it helps.