Stale Statistics...

 Hi all;

I am trying to compare execution plans for some queries both with fresh and with stale statistics.  (collecting fresh statistics dramatically affects the plan and time required.)  I have taken one of our QA databases, and using dbms.scheduler I have disabled nightly statistics collections.  My Question, is 'How do I dertermine when there has been enough activity against the table for teh optimizer to consider the stats as being stale..??  Is there a table or view that tracks this info..??  I need this info to determine how often during a day, the stats can go stale and require a refresh,