how could I say 'How long ago was this photo taken?' I.e. Single event, not duration.

Bonjour Jay !

I'd say the most colloquial way to ask this is the simple:

Quand est-ce que cette photo a été prise ?

Used on its own, how long ago? could be il y a combien de temps ?

However, it would sound a bit clumsy (though not completely incorrect) to use it in a longer question -> Il y a combien de temps que cette photo a été prise ?

I hope that's helpful!
Bonne journée !