You don't get sick and a typical life span would be several centuries. The Culture's 'cities' are its GSV's, most have hundreds of millions of residents or even billions. Rocks consist of a converted asteroid and, like planets, living on one is unusual rather than the norm. The other forms of habitat are all manufactured. The most abundant are Orbitals which are giant rings in orbit around a star.
Plates are similarly in orbit but are a pair of huge plates.
Rings are an even bigger version of an Orbital, instead of orbiting a star they encircle one. Firstly the 'M' stands for Menzies. Banks loathes this attitude and the way many in the literary establishment look down on SF as not being serious writing. This is no longer the case. Look to windward' is the phrase in TS Elliot's The Wasteland that precedes 'Consider Phlebas'.