The perfect time is when you want them. I had my first in the 4th year of medical school! I was shooting for just after graduation, but got pregnant fast. I had to finish Pediatrics and OB-GYN rotations with a baby. The funny thing is that I only had 3 hours per evening to study, so I began studying the first night and studied only that amount each evening, plus more on weekends and I got an A in each clerkship! I had another one in 2nd year of residency and a 3rd one after finishing residency. It can be done : )