'Spoken at length' could have several meanings.

'We have spoken at length on the subject,' means more than one person has discussed a subject.

'He has spoken at length on the subject,' could mean either that a person talked a lot about a subject during discussion or debate, or that a person giving a speech, lecture, written report, and so on, spoke for a comparatively long time, expanding upon the subject rather than just briefly touching upon it or giving only a shortened view of it.