This depends upon how fast you go through things. But, as a 2 person household, we can go through some things pretty slowly. That bottle of mayonnaise can be in the fridge for any number of weeks.
Likewise a bottle of brake fluid or a can of paint will easily sit half full for months, if not years. Knowing when it was opened or, if you really are looking ahead, predicting when it will be of questionable value is something that “future you” will always appreciate about “past you”.
Some things it’s just good to be aware of. I have a “disposable” water bottle sitting on my desk right now where I’ve marked the date that I first opened it on the bottom as January 6, 2016. I use it probably 4 times a week when I work out in my gym and it shows no signs of failing. Disposable indeed.
It takes maybe 10 seconds if you keep sharpies handy in your kitchen or workshop to jot the “opened” or “toss by” date on something. *That* is a tiny investment that yields dividends…