For example 'I haven't done X in an age' vs 'I haven't done X in ages'. Which is correct? Or is it simply a case of American English vs British English?
The plural form is the more common, the singular one is idiomatic in BrE.
Ages (plural), (also an age [singular]) (informal, especially British English):
a very long time:
- I waited for ages. It'll probably take ages to find a parking space. Carlos left ages ago. It's been an age since we've seen them.