I have no experience with -afinils, but it seems to me that there will surely be cases of people who have tried only brand-name (or, alternatively, only generic) -afinil, and therefore cannot accurately respond to the question

In general, do you find brand-name -afinils more effective than generics?

With yes, no, or the same. The correct answer would be "I don't know". If I were taking this survey, I would skip that question rather than try to guess which answer you wanted in that case. But if I were designing the survey, I would go with ChristianKl's suggestion.

I've added 'the same' as a third option to the generic vs brand-name question

I would guess that a majority of the respondents haven't testing multiple kind of modafinil and thus are not equipped to answer the question at all. "I don't know" seems to be the proper answer for them.

What would the response there be, an exact date or n days ago or what?

Both would be possible but I think "n days ago" is more standard. It makes the data analysis easier.