In many of the InfoSec forums I subscribe to people regularly as the "How long is a piece of string" question:
How extensive a risk assessment is required?
It's a perfectly valid question we all have faced, along with the "where do I begin" class of questions.
The ISO-27001 standard lays down some necessities, such as your asset register, but it doesn't tell you the detail necessary. You can choose to say "desktop PCs" as a class without addressing each one, or even addressing the different model. You can say "data centre" without having to enumerate every single component therein.