Alcohol should last as long as it is kept well capped.

Diluted neem looses its effectiveness pretty quickly as far as its active ingredient (azadirachtin) is concerned. However, the oil part of neem should still be effective for a while. You will have really shake it well every time you use it. The oil tries separate from the water when left undisturbed. Oil is organic matter and so it is possible that a diluted version can start decomposing/fermenting. A smell test should let you know if it is OK to use.

Pure insecticidal soap in water should remain effective. I do not know how long but a few months should be fine. I am not sure of household soaps since there are tons of other ingredients in there besides soap. In fact, one should avoid regular household soaps on plants anyway.

Thanks Tropic, that all makes sense. Can understand the neem breaking down pretty quickly, but thought the insecticidal soap might last longer than its label says (use within 8 hours). Probably just want you to go through the bottle faster. I will check if the mix is smellier before I use it again. And add a splash more alcohol because I bet there'll be some evaporation in the cheap little handsprayer.

Do you have a set quarantine routine? I brought in spider mites with my last new plant, which I've just gotten under control. So of course yesterday I saw a nice little rattlesnake plant which I couldn't resist. Didn't think of new bugs until I was back home. Must have had a brain fade when I saw its pretty leaves..

Did not know that insecticidal soap must be used within 8 hrs. So I am guessing those Ready-to-Use bottles must have something to keep it useful. But I think you may be also right that they just want you to use it faster.

I usually make mine in those 24 oz sprayer and usually less than 24oz for quick spray jobs. I hardly ever have leftover. I have way too many plants anyway.

These days I rarely buy plants (refer to too many plants problem). But I do hit it with an alcohol spray if I do buy one. Most of my pest problems happen indoors in winter. To slow it down I will spray all my plants late fall when they are still outside. My favourite go-to is horticultural oil for this purpose.