as for my experience at BCG, you will always need to spend a minimum amount of time in each level, before moving to the next one. Once you have reached the minimum amount of time, you may get promoted immediately, or you may have to wait for an additional 3 to 6 months before having the chance to be considered again for the next level, according to your performance and the slots available. If you do not make to the next level after some attempts, you are normally asked to leave.
Usually, the more senior you get, the more you may have to wait, mainly because there may be less available slots at the next level and more competition.
Time required for each promotion at BCG as for my experience in Italy is the following (it may vary by geography):
- Associate: 24-36 months
- Consultant: 24-36 months
- Project Leader: 24-30 months
- Principal: 36-60 months
After the Associate step, you may have to do an MBA, or be required to spend one year as Senior Associate before becoming a Consultant.
Hope this helps,