Hi Doris: I'm not an expert on metal flues, but I have worked on a few.
The life of a metal flue depends on what it is made of (stainless steel would be best), what kind of wood has been burned in it, how often it is cleaned and what kind of fire box is being used. The hardest firebox on a metal flue is the air-tight wood stove.....these tend to operate at lower temperatures than open fireplaces, thus have a greater tendency to deposit tars and oils on the inside of the chimney. Burning softwoods generally will generate more creosote (tar) than hardwoods.
Your best bet would be to take a section out of your flue and inspect it. Thick or clumpy deposits of black stuff (tar) suggests that the liner will need to be replaced.
Another word of caution. You don't mention who said your flue is a "fire trap"....but this sounds like a flue-replacement salesman.