Google published an interesting video yesterday detailing how they prioritise spam reports. Basically they look at how many users are potentially affected by the spam and order the reports based on urgency.
The spam report form asks for the keywords, query and web page so Google can calculate the number of users exposed to the “spam” site on a daily basis for both the target query and a list of related queries as well as overall traffic numbers.
In the video Matt Cutts said that lower priority spam reports for small sites/unpopular queries would probably be dealt with in the next round of algorithm updates rather than right away.
The takeaway here is that unless you are reporting a small site ranking for a major query then it’s unlikely you will see that result being moved very quickly.