how long should replaced spark plugs and wires last before needing to be replace on 1999 Ford Expedition

had the plugs and wires replaced 30 months ago and they say I need them again, does that sound right?

Asked by for the 1999 Ford Expedition
You say it runs rough at start up, does it clear up after? Where did the replacement parts come from?
Most premium parts have a lifetime replacement warranty! If it runs ok after it warms up then it aint plugs or wires.
No way it SHOULD need those parts again at that mileage or time period. IF prem. parts were installed.
that's what I thought, thanks for your reply
2 more answers , 2 more comments
depends on how many miles you have put on it
Does vehicle have a check engine light on? I assume vehicle is 4.6L- 5.4L has coil over plugs. If you didn't put too many miles on and is misfiring on particular cylinder suspect coil if 5.4L. Give us some more info so we can try and help- who's they and reason you took for service?
check engine soon light came on and is running rough when first started. 4.6 engine and I put on abour 20,000 miles since having the wires and plugs replaced at the same repair facility.
Did they give you code from engine light?