Well, let's see here. There are several different inspections and overhauls that need to be done at various hour limits. The powerplants, much like virtually everything on the airplane, and the airplane itself, have hour limits. After X number of hours of operation, the powerplant, or pump, or whatever is considered expired. Unsafe. Unairworthy. In aviation, just about everything has a time limit. The expression "timed out" is used a lot. Think of it like this: Take a thin piece of metal in your hands and bend it back and forth...the question is how long can you do that before it snaps in two?? Time limits for aviation components are set with safety and performance in mind, best of both worlds, so to speak.
The life of a powerplant in plenty long. As for how long...well that depends on the manufacturer. The engine will be overhauled several times during it's life, and see service on many different airframes. As I said above, an aircraft comes in with an overhaul due on an engine, or has a problem, they will change the engine out and return the airplane to service, while the engine that needs service is placed on a stand for mx. Once it's green tagged, it will wait until the next aircraft with that type of powerplant needs a change.
Most MD-80s use a variant of the P&W JT8D powerplant. Here is a pic of a JT8D-219 "out of the box":
http://www.flightglobal.com/assets/getAsset.aspx?ItemID=23540The JT8D was used on several aircraft types, including the MD-80, DC-9 variants, B737-200, and 727 variants. Here is a JT8D powerplant on a Boeing 727-200F with the cowlings removed:
http://cdn-www.airliners.net/aviation-photos/photos/4/9/4/1002494.jpgAnd all cowled up on a B737-200:
http://urawa.cool.ne.jp/serippe/JapanTransOceanAir/NUB32JA8366OKAJT8D17EngineUp.jpgHere is a spare JT8D ready to be installed on a needy aircraft:
http://farm3.static.flickr.com/2033/2071905553_2f0015993d.jpg?v=0And here is another spare:
http://upload.wikimedia.org/wikipedia/commons/3/36/JT8D-9A-071207.jpgAnd I just like this pic. JT8D on the bird with the lower cowling open. I love the Super 80 logo on the upper cowling!:
http://cdn-www.airliners.net/aviation-photos/photos/7/2/3/1019327.jpgJust for kicks, here are some other powerplant types:
Here is a GE aircraft engine completely off the airframe and out of its cowlings:
http://blogs.aviation.ca/media/ge90+1.jpgHere is a GE powerplant getting an inspection:
http://www.businessfacilities.com/blog/uploaded_images/GEengine-763893.jpgAnd here is some powerplants...past useful life...
http://img.alibaba.com/photo/11181800/Airplane_Jet_Engine_Scrap.jpgHere is an uncontained engine failure on a Delta MD-80:
http://upload.wikimedia.org/wikipedia/commons/d/d5/DAL1288a.jpgProbably a longer answer then you were looking for, but I hope it answers your question!