I have the entry level 21.5 2012 model myself. Honestly, I never intended to keep it more than a year but Apple was slow to upgrade so it turned into 4+ years. In my case with the integrated video, 8GB RAM, (never used the internal 5400RPM hard drive), it's a bit long in the tooth, esp editing pictured in Lightroom. So I do plan on upgrading to a 2017 iMac as soon as they start getting discounted. I noticed Adorama is already $20 off on top of no sales tax so it's starting already. I figure once all the main resellers get them in stock the price will drop $50 to $100 off and I'll jump.
You have a more powerful machine than I and I don't know how you use it, but if you notice sluggishness with video or photo editing or even everyday scrolling and program launching then an upgrade is something worth considering. If you just have CPU envy then flip a coin. The great thing about Macs is the resale value really helps soften the $ blow when upgrading to a newer model.
