I also feel like the CGM lasts best on my upper arm, though I do also do a stomach site from time to time. I have a routine I’ve gotten down to make the adhesive last, and it works well for me.
To start, I always use SkinTac (available from Amazon in either liquid or wipe form) under the sensor adhesive. Basically wipe it on in a circle (leaving a circle in the middle where you actually insert the sensor), let it dry, then apply the sensor over it. This has really helped me.
Then, as soon as the edges start peeling up, I put a GriffGrip on over the top of it all (once again with more SkinTac underneath). The oval GriffGrips are the best, as any of the “fun” shapes with points and stuff actually peel off a lot faster.
Though you are supposed to change the site every week, I will admit to frequently using mine for 3 weeks. This system (normal adhesive for about 1 week, added GriffGrip after that) helps it stay on nicely that whole time, including during swimming and exercise.
If you use SkinTac, make sure you also order adhesive remover wipes. It’s pretty strong stuff. Good luck!