I was cleaning my mixing cupboard earlier today and found an old jar of nic that's at least 2 years old that I stopped using cause I spilled some concentrate into it. I'd guess 40 to 50ml in the jar and it still looks like one homogeneous liquid, no layers or anything. Although it began life mostly clear and now it's turned morning piss yellow after a heavy nights drinking. The jar is staying sealed and going in the bin, don't want to know what it smells like.
I think once the nic is properly mixed into the VG, it's unlikely the two will "unmix". That said though I do give everything a quick shake (concentrates included) before I use it, more out of habit rather than necessity. I usually leave the nic I'm using in my cupboard and the rest in the freezer in 120ml bottles. And when my cupboard nic is running low, I'll take a bottle out the freezer and put it in the cupboard so when it runs out, I've got another bottle at room temp ready to go.
I think nic will only turn yellow and lose strength with age as it gets exposed to heat and air. Although you said you keep yours in the freezer, so it should last at least a couple years. It is possible that the 3mg nic you bought from the shop was mixed wrongly if they do their own DIY
