That has proven to be a difficult question to answer accurately because the Solution Manager scope has been expanding over the years to cover many different functional and solution support areas.
One of the best resources to help understand the overall size and scope of Solution Manager is the recently published SAP Solution Manager 7.2 Adoption Framework. This lengthy document describes 40 different functional areas in Solution Manager 7.2. SAP also provides some rough guidelines regarding estimated efforts to implement the functionality. What struck me was the overwhelming amount of effort required when you take a closer look.