I believe it took them a few years. They still had not recovered all their organic traffic when I worked on them but we did see a sharp increase in conversion rate. Revenue actually improved I think Moz has a case study out from when they changed from Seomoz to just Moz.
I would just make sure the client understands improving organic traffic after the rebrand will be a steady process. You could look at supplementing organic with a PPC campaign or venture into social media advertising and blogging until you see improvements.
Hope that helps some.