This really depends on how well they are looked after, both when on display and when being stored. If treated properly they can last you many Christmas'.
Garlands are excellent because you can put them wherever you like, traditionally they sit on a fireplace, however they look great on windowsills, stairways, shelves or on the top of cupboards.
Artificial greenery is not really made for the outdoors and the elements will affect it's longevity. For the best results we suggest using them indoors or under covered areas. We never advise using artificial wreaths or folliage outdoors if they have lights attached.
A garland i measured lengthways along the wire on which it sits.