When does a neologism cease to be a neologism?
Solution 1:
Neologisms can be divided in three categories/phases:
- Unstables: Those that belong to this category are the newest ones, as in being just introduced, or that are used by few.
- Spread: As the name says, they are spread enough, so they are used by a wide number of people, but are not accepted as "normal" words yet.
- Stables: Those who get accepted by the public audience and start to be used like other words become stable.
Anyway, although we can say that when a word goes inside the Dictionaries, it is not a neologism anymore, if it's being used so commonly and widely, it will be considered a normal word like others, and this is what counts.
Sometimes, even when experts or linguists don't agree on some neologism, for various reasons, what really matters is the common usage and the acceptance by the public audience, as you could also see from the three categories/phases I wrote above.
If the event/situation the neologism describes cease to exist, the word will stop being used and therefore disappear. This is to answer to your second question.
Solution 2:
As a rule of thumb, if it appears in a general interest broadcast or publication without being accompanied by a definition, it's probably not a neologism anymore.
However, this doesn't mean that a word accompanied by a definition is necessarily a neologism; it may simply be obscure or unfamiliar to the target audience.