Execute the following two blocks of code separately from the developer console. The system will create two jobs, one which runs every 0th minute (1PM 2PM 3PM etc) and one which runs every 30th minute (1:30PM 2:30PM 3:30PM etc)

ClassName gp = new ClassName();
String cronStr = '0 0 * * * ?';
System.schedule('Job which runs at every 0th minute of hour', cronStr, gp);


ClassName gp = new ClassName();
String cronStr = '0 30 * * * ?';
System.schedule('Job which runs at every 30th minute of hour', cronStr, gp);

Problem:

You get this exception message on a simple aggregate query like this.

Select Status from Case GROUP BY Status - FAILS

You try to use a limit clause but the query still fails.

Select Status from Case GROUP BY Status LIMIT 1000 - STILL FAILS

 

Reason:

Even though you may use a limit clause, aggregate queries will have to touch all records in the database. If you have less than 50000 Cases you would not receive this error message.

Solution:

Couple of probable solutions.

1. Try to limit query results using a WHERE clause.
2. If using this on a visualforce page and you do not perform any DML (insert, update etc) from the page, set readOnly=true on the apex:page tag.
3. If using this on a visualforce page and you need to perform DML operations, use @RemoteAction and @ReadOnly on your apex class and deal with the results using javascript. Example link. 

Make sure the following box is checked for the user who runs the flow. This box is unchecked by default.