There doesn't appear to be a specified time, though this OP got a response within 5 days, I would guess that Google will get back to you within a month.
The following two articles might be of interest/helpful
There is also a useful post on Google groups from Google's own John Mueller:-
While we have just recently started sending out these messages, they may apply to issues that were already known (and affecting your site's
standing in our search results) for a while.
If you receive a message like this, and you wish to resolve those issues, then I'd always submit a reconsideration request after having
done so. In some cases, you may not be able to resolve all of the
issues - if that's the case, then it's important to us that you
document your efforts (you might even link to a Google Docs file if
needed). It's important to our team that it's clear that you have
taken significant effort to resolve all of the problems in that area,
and that they can trust that these kinds of issues will not come back
in the future.
In situations where an algorithmic adjustment might have been made, you're still welcome to submit a reconsideration request. It doesn't
cause any problems to do that, so especially if you're unsure,
submitting one is a good way to be certain.
Regarding the age of the unnatural links, I'd work to have them all removed, regardless of the age. For instance, in the general case
where a site has been buying links for 2 years, it would be a good
idea to go back that far.
If you're serious about your site's standing in our search results, then I'd strongly advise not to try to wait the penalty out. These are
generally not issues that expire after a few days, they can affect
your site's standing for quite some time.
For what it's worth (and totally unrelated to the above), I also
noticed that you have a lot of URLs that are being blocked by the
somewhat generic "Disallow: /*?" directive. It looks like you're using
that as a way to handle duplicate content and/or canonicalization. In
general, we recommend not doing that, as it makes it impossible for us
to recognize that these URLs are leading to the same content, which in
turn means that any links (including internal ones) to those URLs will
be "lost" instead of being forwarded to your preferred canonical. In
your case, we attempted to crawl about 63k URLs that were blocked by
your robots.txt file in March (compared to 61k URLs that we were able
to crawl normally). Our Help Center has some more information on how
to handle canonicalization at
it's also worth reading this other article from Barry Schwarz.
Hope this helps you out.