I keep using mine and have never had fuel issues.
For how long after opening?
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I've got a place that will require a lot more OPE, so in the future I will go through Stabil a little more quickly.
Literally years. I bought a big bottle and use maybe 15-20 gallons of gas a year. As mentioned, no fuel issues in 2 or 4 stroke equipment, including seldom used stuff and stuff that has sat with fuel in it over winters.
Keep it sealed tight, keep oxygen out, and there's not much to worry about imo. Especially if you'll be using it faster in the future.
I suspect the fuel life extension diminishes as the active ingredients get consumed from atmospheric oxygen. But hrs the longer you're truly trying to stabilize fuel for? A few months over the winter? You don't necessarily need the full stability life.