Hey oldman,
Here's the scoop:
Step I: Usually taken after 2nd year. Most schools have a month (we had 1.5 months) off before clinicals. You can schedule it at any time since it is on the computer. This is the most important step for residencies, so study hard. I studied about 3 weeks 4-5 hours a day, many of my classmates did more.
Step II: Some say take it Fall of 4th year, but most take it spring after interviews and match. It is not as important as step I. However, if you didn't ace Step I doing well on step II might help your cause if you take it early. One option is to take it in November and do not release scores automatically to ERAS (residency program). This way, if you do well you can tell interviewers about it but if you don't it won't hurt you. Lowest stress option is to take it late like I did in February. I studied 2 weeks about 1-2 hours a day and 5 hours the day before and did great.
Step III: Taken after intern year. Usually some time in second year. My friends in residency say they studied maybe 2 days.