My usual question on the interview is to implement an atoi() function. All I care is that a person doesn't do stupid shit like switch case for characters or a strlen() for iterating through the input. It really is a 10 to 15 minutes task and boy it shows whether a person can write code. Usually then I continue with "how'd you test it?" question, then move on to multithreading and a quick design question. All in all I spend maybe 30 minutes to tell if I'd make a hire.