Between these two support articles, I think they’ve answered these questions pretty well:
https://support.formlabs.com/s/article/Choosing-the-Right-Material?language=en_US
https://support.formlabs.com/s/article/Resin-Care?language=en_US
Long story short, resin lifespan is 24 months, but environmental/storage factors can and will change this. They can’t give you a hard timeline, because if you store your resin in a covered tank out in the open with ambient sunlight, vs pouring it into an opaque bottle, the lifespan of your resin will change drastically. Nearly all these resins have something suspended in them to give them their mechanical & visual properties. These additives can settle, that is why the cartridges tell you to shake thoroughly, why the tank has a wiper, why they all have different settings. If you have scraped and discarded some of the settled material, you have changed the formulation of the resin in the tank, and it should be discarded. If it has clumped up, solidified, or just completely unable to be mixed back in even after heating, then yeh, it’s no good anymore.