The process took 1 day. I interviewed at Group One Trading (Chicago, IL) in Jan 2010
Interview
Group One has a very informal hiring and interview process. Some candidates are referred to the HR department (now down to one person after the downsizing) and some through school's career services department. Due to the financial difficulties that the company has been having they have cut down on the number of employees as well as their campus visits. A few trainee's in my class had phone interviews as well as in-person interviews, while others simply had phone or in-person only. I also had one individual who went through a phone, one-on-one interview and then a group interview.
The two most important interview questions to answer are why you want to be a trader and the mental math questions. They are looking for smart, driven and focused individuals who have a vague idea of what they are applying for. A solid understanding of options is definitely a plus. The training program is really hit or miss. You spend roughly 2 weeks going through Sheldon Natenberg's book and then the remainder of the training period (well over a year, do not believe what HR tells you) consists of mock trading on a white board. Their new accelerated training program consists of mock trading sessions twice daily so I hope you enjoy plenty of mental math each day. The training program changes each year and there up until recently has not been a full time trainer. Even with a full time trainer classes are skipped and cancelled fairly frequently. Due to the training program or other circumstances the retention rate at Group One is between 10-20%. Approximately 1 or 2 in 10 will make it through the three stages of the training program and be allowed to trade.
Bottom line is even if you make it through the initial interview and are offered a position you are still being interviewed on a daily basis. If your are performing poorly in the mock trading sessions, not speaking loud enough, or dont show enough enthusiasm for trading you will be let go immediately.