I kept stats for a couple of years on large numbers of quant financial interviews. Typically up to 15 roughly 30 minute interviews per hired candidate. The probability of getting hired seemed to go to up significantly by interview number until about #7. After that it was flat until the last guy, probably because nobody wanted to say no and contradict the first 10 people. The extra interviews were useful, however, as they helped establish consensus and buy in among the team