When to replace 18650 batteries?

I'm vaping since almost 4 Years on the same VTC6 batteries. I have 3 x 2 in rotation in two dual 18650 mods. They are still in good shape. Two pairs of them I re-warped.

I wonder how much capacity they lost over time. Would you say it's time to replace them, or keep vaping on them since they are in good condition?