how long back it happened

[tufguy]

My question is in regard to "how long back". Have I used it correctly?

I don't remember "how long back it happened" but it seems it happened aproximately fifteen years ago.

 

[Rover_KE]

Change 'back' to 'ago'.

In another context, you could say 'What you consider historic depends on how far back your memory extends.

 

[jutfrank]

I don't remember how long ago it happened ...

[cross-posted]