Is there any difference, whether in terms of usage or meaning between the two? For instance: How long before/ till the alarm goes off? How long till/ before you get here? How long before/ till this term ends?
To me (native American English speaker) they have the same meaning, but "before" sounds perhaps regional or outdated. It's totally grammatical and comprehensible, but I wouldn't expect to hear it often in Seattle.
In general, this construction, as compared to:
When does the term end?
What time do you get here?
has a slight nuance of "I think it's taking too long for the event to occur." If someone asks me:
How long till you get here?
I will probably hear, "I'm tired of waiting for you; why is it taking so long?" But it depends on the context and tone of voice.