It's entirely up to you. Book-store, bookstore, book store; all OK. With noun + noun combinations, if people understand it, it's correct. If people go 'huh?', it isn't.
I'd have thought it would be 'Endtime'.
You're right that it might be better to hyphenate it when using it as a modifier itself; three nouns in a row can get confusing. In a cat painting contest, who or what does the painting?
Indeed, jusork: Merriam-Webster ( or is that Merriamwebster ? ) says 'endtime' is not in the dictionary. It suggests Antrim or enthymeme as possibilities. [The Ant-rim, apparently, is at the edge, the rim, of the United Kingdom of Great Britain and Northern Ireland and largely populated by those anti the place being part of the UK, hence the name. Enthymeme is a defective syllogism: we all know how painful that is ]
