Why assess? To understand strengths and opportunities for improvement? To help decide whether your team/organisation is ready to implement a technical practice, e.g., TDD? To benchmark against best practice?
First step is defining clear assessment goals and focus - engineering practices, processes, both. Our assessments can range from anything from a few hours to a few weeks, depending on the scope and scale.
We have assessed using a number of best practice models/methodologies including: CMMI, ITIL, SPICE and Agile.