I've had a discussion with a friend because of the following question and its answer:
Q: How often do you visit your grandfather?
A: Last week.
As far as I know, "when" requires a specific time, while "how often" has to do with the frequency but my friend insists that there's no difference and they can both be used the same way. I'm looking for detailed information about this?
Thanks.

You are right.


"When" requires a specific time:
Q: When?
A: Last week. [Correct]

"How often" means "how frequently".

There is no need for a more detailed answer. I can tell your friend that both "when" and "how often" require answers that are both adverbs of time, but the commonality ends there. All of the following are adverbs of time:

now
yesterday
256 times a second
at 6 o'clock
on Christmas Day
Last week
3 times a year


It is interesting to note that "on Christmas Day" could be an answer to both "when" and "how often". However, that is only because it is a specific time and a frequency. So, there is one point for your friend. In general terms, you are correct.

Thanks a lot.