It is a hard line to define IMO. If the question has an answer, IMO, answer the question and redirect to the appropriate support channel(s).

If the question is clearly specific to mint / debian / fedora / whatever, explain the question is best answered by alternate support channel and close as off topic.

It sort of depends on 2 issues:

- How friendly do you want to seem to non Ubuntu users ?
- How tolerate do you want to be of general linux questions that apply to any distro vs redirecting to superuser ?