I had a tune-up between 45k and 49k with new mopar wires and plugs...Nothing aftermarket. Someone else might say go outside Mopar, but I just stick with what it came with. ( Mine is a non-turbo convert, as pictured below ) I'm sure it'll last as long as you want given it's matained well and regular oil changes. Also check the pcv valve, I had mine replaced at 22k, I forget why but it needed it at the time ( I have 52k on mine right now.)
PT's arn't known for excellent gas mileage, Mine is like 19 city and 28 Highway, or something like that, maybe 26, I'd have to go look at the original window sticker, but I'd def. get the 30K Mile service because I noticed my car started to get kinda sluggish before I did the tune-up and fuel injector cleaning. Shop around though, dont go to the dealer because they were going to charge me between 550 and 600 before taxes to do that, but for my luck my boyfriend's brother is a mechanic and I only paid for parts. Also check tire pressure, under or over inflated tires can cause bad mileage.
They are fun cars, especially for summer crusin' ( atleast for me, LOL ) but it'll last as long as it's taken care of
Hope some of that helped :-)