This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
You'll run your application and its query under expected load. Then you'll create an AWR report. If your query doesn't appear anywhere in it, it is probably not worth tuning it.
Other than this, it is hard to say. A query which is run once a day outside business hours may not significantly impact the database even if it consumes a lot of resources. A query that is run once (or more) a second may impact the database heavily even if it consumes just a little of database resources. Also, if there are normally many applications running on the same database (a very usual setup nowadays), assessing the query performance in a test/dev environment that doesn't have the load from the other applications can be very misleading.
You can also do various things while you are developing your query to make sure it will perform well e.g. use EXPLAIN PLAN to check it is using indexes correctly or to figure out where you might want to add indexes to improve performance.