Does anyone know the shelf-life of most acrylics and how long their colour richness and quality will last form the date you’ve opened them?

I’ve had a set of Dalery-Rowney System 3 acrylics for over 2-3 years now and originally, aside from just customising ponies, I found them great for painting artwork. They seemed to blend and mix well and the colours were beautifully pigmented.

Today some of my colours are still strong, but I found my white and black to dry on the page like tar. They dry incredibly quickly and getting the colours to blend together is proving to be a pain, as no matter how much paint I apply (and no matter how quickly I follow it up with the next colour) they just turn flat and don't blend. My yellow also seemed a bit watery when I first used it and I noticed it was a bit more opaque and weaker than it used to be.

Although I’ve had them for a couple of years I’ve always stored them carefully with caps on. I only noticed the difference in quality recently when I tried out some cheap student grade acrylics which actually blended more easily yet were just as pigmented! That’s what makes me wonder if my System 3’s might be kicking the bucket already, but I can't really tell. I need to try all the colours and test out blending more to see how they are.

Anyway, how often do you guys find you need to replace your paints when their colour has died down or once they've dried out? Do they last you only months or several years and out of curiosity has anyone else had these problems with System 3?

Sounds like it's probably time to retire those black/white paints of yours.
I noticed that after about 3 years of not using my acrylics, they turned out lumpy when I tried using them again. X____X I had to retire/replace my supply....but the smoothness from the fresh acrylics, was worth it.

So I'd guess that the shelf life of acrylics is probably about 2-3 years.

Some of my acrylics are ones I used at school 15 years ago, some need a bit of mixing medium but the pigment is still fine. I use it for background washes and things when painting. The black goes lumpy after time for me too, but I don't often use black so it isn't an issue!