The database will get larger as you include more data. Unless the storage is very small you shouldn't need to worry about this so much unless it went from 300mb to 600mb shortly?

Should I plan to backup the log files every 15 min? (6/15/2013)

WHERE recovery_model_desc != 'SIMPLE'

If you run this on the database does it return any rows besides "model"? If not then you don't need to worry about backing up the transaction logs. If there is more then you need to backup those transaction logs but you won't need to do it with that much frequency...I'm guessing your database isn't very busy? 500mb would say it isn't. Running them every hour should be enough; having said that it doesn't really matter.

Watch the videos on YouTube for how to use maintenance plans. You can use this to backup your database (nightly full backup) and your transaction log (every 15 minutes)