No-one can answer that question. No-one knows if Blizzard needs to keep a record of your botting actions, and if so, for how long. Maybe they need to log your behaviour for at least x hours to be fairly sure you're botting. Maybe they know it in a matter of minutes or even seconds. They might even detect the bot the moment HB attaches to your WoW process. No-one knows. The ban reports are also very diverse; some botters who bot 24/7 on multiple accounts got banned, but people who only used fishing bot in their own garrison for 15 minutes also got banned.
No-one knows. Yeah, well, Blizzard obviously knows, but I doubt it you can just call 'em up and ask it