That's a very personal question depending on how much you care about having the latest and greatest vs. saving the money.
I'll put it this way. I currently have a 50PZ85U that I got nearly 6 years (5 years and 8 months) ago. It has 8850 hours on it and is still going strong. I don't think it needs replacing, though clearly the black levels aren't great compared to today's plasma offerings, but still better than a lot of today's LCDs. I just ordered a 60ZT60 as a replacement because Panasonic is going out of business (Samsung F8500 would be my next choice). Do these new sets totally blow my old set out of the water? I'd say they are a solid improvement, but if Panasonic or Samsung were definitely going to keep making plasmas for a few more years, I would have waited. Disclaimer is that I care more about audio quality than video. For me, the 50PZ85U is still adequate, especially if you don't watch in complete darkness.
If I were you, I'd wait 2-3 more years, given that your TV isn't that old. Each year is only an incremental improvement. Who knows, maybe OLED will be cheaper or local dimming LCDs will perform better than your plasma down the road. Agree with The Void 100%.