The EZ-Lam product is good stuff. I started using it instead of a prior product that required thinning to work well when using .58 oz/yd2 glass cloth. The thinning caused additional issues with curing--problems that you won't have with EZ-Lam right out of the bottles.
I have purchased at least three kits over the years. I keep it handy so when I am motivated to build, I don't have to wait. So far, it has been usable up to about 3 years. A batch last year didn't cure, so I ordered more. Problem solved. The latest kit came with stickers that gave the manufacturing date and expiration date. They are stating a one year shelf life, which is pretty good.
The part A may turn cloudy and thicken when the shelf life is up. You can warm the bottle in a pan of water and it then turns clear and looks normal, but you are at the end of life, and even if it cures, may not do so reliably. It also will not be as strong, but as Brett points out, for model building we hardly use the strength of the product. The part B hardener turns a bit darker--or else the bottle is getting stained. It does get sticky on the outside with age. Not from drips but from migrating thru the plastic.
If you have had epoxy applied for 48 hours and it has not fully cured, it probably never will. I have put items into the oven that were glassed using old epoxy and they did not fully cure. So removal/cleanup is the next step. If you are lucky it did not even begin to cure and you can scrape it off and solvent wipe. A bit of time to outgas and you are ready to start again. For a glass job, it is pointless to try sanding partially cured glass/epoxy. I found that I could peel up an edge and using a roller to wrap the fabric onto, continue to roll and peel. This was easy once I got it started. The parts were solid parts though. I would hate to try this over a planked foam wing! Maybe it would work in fairly narrow strips.
To make sure I get the proper mix, I use the small "K&B" cups and weigh both parts out on a scale. (Triple beam balance, actually.) I never mess with the ratio hoping to shorten or lengthen the pot life. Unless it is old--and I know it is old before I mix it--it has never failed to cure satisfactorily. I taper the tip of the popsicle stick so I can mix right into the corners of the cup.
Of the different epoxies that I have used, this one sands the best, isn't too brittle, and is thin enough for model work (laminating and glassing). I have also heard good things about West Systems. I believe its biggest claim to fame is that it will cure at fairly low temperatures. This is really important to boat builders working in unheated sheds, which I think was one of the original markets for the product. The homebuilt airplane guys use it a lot, too.
Dave