First off, this is NOT to be confused with the following question:
What could happen if I used 4 cycle fuel in a 2 cycle engine?
2 cycle oil is used for my weed eater. There is no way I could use a whole gallon in a year.
Sometimes I run out of gasoline for my 4 cycle lawn mower, and that 2 cycle mix fuel is sitting there getting really old.
Best Answer
It will smoke badly and may indeed foul the plug(s). It will be less of a problem if you only use a SMALL amount of mix fuel in each tank, so the oil is more diluted.
IMHO, IME, if you use THAT little fuel in your weed whacker, you probably should sell it and buy an electric one, either corded or rechargeable.
If you don't want to do that, then you should invest in some good liquid measures that you will NOT use for anything else, and mix only a pint or quart of mixed fuel at one time - or purchase "canned pre-mix" fuel which typically comes in sealed metal quart cans - available in many places.
Edit WRT comments: I would personally choose an inexpensive graduated cylinder (or two) for the mixing - far less likely to be repurposed to food use by someone that does not know better than a tablespoon - also very easy to get a direct ratio in small quantity. But...2 Tbs in an ounce, 16 per cup, 32 per pint, 128 per half-gallon. So a pint of 32:1 mix is easy, but all my stuff is 50:1 mix. 256 tablespoons per gallon at 50:1 works back to one Tbs per 0.195 gallon (on a modern digital pump readout.) 40:1 is 0.156 and 32:1 is 0.125