Longevity of garden chemical products

DrMac

Very few, if any, products have a sell by date, so generally I apply a rule of thumb - wet products a couple of years, dry products 4 years in air tight containers or if in sealed packs, as long as they look OK. I keep all in the gardens shed so generally kept at a reasonable temperature. Disposing of partially used containers is always a problem, particularly if one does not live near a council skip area, another reason for trying to use them up fully.

I would be interested in other views and in particular about some specific products.
Tomorite liquid feeds
Powder fertilisers such as Growmore, Chempack, etc. As these are stable chemicals I would have though that as long as kept dry they should last for ages.
Insecticides/weedkillers (use of which I try to keep to an absolute minimum, before anyone takes me to task for doing so, but as I get older and less able, digging weeds out is sometimes not an easy options). 

Fairygirl

I often have tomato food left over and just use it the following year. I can't say I've noticed any great difference in performance. I don't just use it for tomatoes though - it's great for things like clematis and sweet peas. Anything which is a heavy flowerer.
I occasionally use the slow release granular feeds for the aforementioned sweet peas as they're mainly in pots, and again, I don't see a poorer display. I've had the same jar for about three years as I don't use a huge amount. 
The newer weedkillers don't contain glyphosate, so they're less damaging. I only use them sparingly, in places that any persistent weed is hard to remove by hand.
I can't comment on any of the other products you mention as I don't use them 

I don't use any insecticides .  

philippasmith2

Tomorite I find will last at least a couple of years and probably longer from when purchased.  Sealed top and not left in the sun/heat.
Growmore or similar "dry" feeds - as you say -  for ages if stored correctly.
The only problem in really knowing how effective they are after a period of time is that unless you can replicate the exact conditions year on year, it would be somewhat difficult to prove one way or another.
Buying the amount you are likely to use over a couple of years would make sense I guess.  Otherwise just using up what you have left over  would at least avoid having to "dump" it elsewhere.