Guide to check how often use your users your search pages
 

1. Google Analytics > Content > Top Content > below enter Filter Page:search.php
2. above the statistics, click on Pageviews>Graph mode: Compare to site

Now when you click on any date point in the graph, you see a % statistic. Enlarge your time frame and group it by months.

2008: Around 0.7 %
2009: Around 0.6 %
2010: Until vb4 migration I had 0.6 % After migration 0.1 %. Currently it's around 0.3 % rising to 0.4 %