It just depends.
You can query your database as often as you can, however you may meet efficiency issue.You can just write a script which just query your database and set a clock to make that script just run for sometime (30 seconds for example). You count the total query, and output the count into file.
Use this method you can test the max qps of your database with that specific query.
Normally, you should reduce the frequency of every database query, cause the database query may be the bottleneck of your script speed.