Dynamic Sleep Activity to Schedule Runbook

I was designing a workflow where there was a Main Runbook that called RunbookA and Runbook B. 

The catch was RunbookA was invoked at 3pm and looped for 3 hours until a file was processed successfully (so there was no standard end time).  Also, RunbookB needed to run at 6pm only if RunbookA was successful.

So I needed a way to determine/schedule RunbookB to run at 6pm if RunbookA completed at 3pm or 4pm or 5pm (you get the point).  So I used PowerShell to create a dynamic Sleep based on when RunbookA completed to invoke RunbookB at 6pm.