The book that's the focus of the spell scribe's binding fills with the subject's memories upon the spell's resolution. Were there some lag between the subject failing (or forgoing!) its saving throw against the spell and the focus book filling with the subject's memories, the spell would mention that. As it stands, the spell doesn't mention that, so the book fills up immediately.
The spell's duration may be permanent—that's how long the spell's effect endures—, but the spell's casting time remains 1 standard action, and, at the end of that casting time, the spell's effect comes into existence.