From interviews by folks on Wallstreetoasis.com, this is my understanding of the hours. Some days they may have 2-4 hour breaks (gym time / internet time), then have to crunch in the evening when a client, or an internal-facing client (such as a Managing Partner) comes back.

In my short time at a small consulting firm (Management / C level), this was also the case.

Many times my long hours were due to "deliverables" (PowerPoints, documents, etc.) being in a choke point based on an Analyst, Consultant, etc.

E.g., waiting on numbers to finish a presentation.

However, much of the time, their is ALWAYS something to do, and much of it is very much face-time (butts in seats = working).