The University of Illinois Extension service says you should harvest beans when the pods are firm and have reached their full length.

VOA: special.2009.03.24

He likes to be in situations where he can talk at length about what the parts are of an issue."

VOA: standard.2010.01.16

In terms of the time length, I think you are looking at months, not weeks."

VOA: standard.2010.03.12