I would be surprised if a well constructed 2 hour test wasn't able to give you enough information on their structure and approach. 4-6 hours probably results i

I would be surprised if a well constructed 2 hour test wasn't able to give you enough information on their structure and approach. 4-6 hours probably results in the candidate filling in a lot of basic details to get something to actually work that will tell the interviewer little more than the 1-2 hours spent implementing the core parts of the task.