Hi Nicolai, 

The Apex-Code should be like this: 

public with sharing class GetFirstFromCollection {
	    @InvocableMethod(label = 'Get business hours' description = 'Get business hours')
	public static List<Results> execute(List<Requests> requestList) {
		//	List<SObject> inputCollection = requestList[0].inputCollection;
		// Get the default business hours
		BusinessHours bh = [SELECT Id FROM BusinessHours WHERE IsDefault = true];

		// Create Datetime on May 28, 2013 at 1:06:08 AM in the local timezone.
		Datetime targetTime = Datetime.newInstance(2013, 5, 28, 1, 6, 8);

		// Find whether the time is within the default business hours
		Boolean isWithin = BusinessHours.isWithin(bh.id, targetTime);

		//Create a Results object to hold the return values
		Results response = new Results();

		//Add the return values to the Results object
		response.outputMember = isWithin;

		//Wrap the Results object in a List container 
		//(an extra step added to allow this interface to also support bulkification)
		List<Results> responseWrapper = new List<Results> ();
		responseWrapper.add(response);
		return responseWrapper;
	}


	public class Requests {
		@InvocableVariable(label = 'Records for Input' description = 'yourDescription' required = false)
		public List<SObject> inputCollection;
	}

	public class Results {
		@InvocableVariable(label = 'Records for Output' description = 'yourDescription' required = true)
		public Boolean outputMember;
	}
}

Maybe you should change the name of the class and variable to be meaningful. 

In the Flow you add an Apex Action then you select the invocabel method "Get business hours". After that you can use the outputMember in the flow!

Regards,
Omar