I'd like to echo what Team Sand Dollar wrote about this and add the 
following:
 A good cache ought to stay active as long as it remains active.  In 
other words, if people are still searching for it, leave it out there. 
 If the searches are few and far between, or if maintenance on the cache 
isn't worth the effort, then by all means archive it.  As for "keeping 
it fresh", I think about all the newcomers to geocaching.  All the 
caches are "fresh" to them.  Just because all the current regulars have 
hit a cache, doesn't mean there won't be others in the future who will 
be surprised or fooled by that clever hide and enjoy it just as much.  I 
do agree that some caches ought to be archived--  I saw a number of them 
this weekend.  They had an inch or more of water in them and they were 
so overgrown with mold and mildew inside that I couldn't even recognize 
the contents.  The log books were mush.   They obviously are not  being 
maintained and they have had few hits in the past months. But I 
digress....   I've been caching for just under a year and I hate to 
think how many cool caches I would have missed if they had been archived 
after only 6 months of activity.

-- Sprocket

>I would say some caches may require a time limit but usually those will
>disappear on there own.  Others should be left alone if they are still
>getting hits.  Others should never be removed on purpose.  If a cache is
>good, leave it. If it gets a good response, leave it. If it's so-so then a
>limit is ok.  Each cache should be evaluated for it's on merits an treated
>accordingly.
>
>Team Sand Dollar
>