When do merchants restock in Might and Magic VI: The Mandate of Heaven?

What does not work:

• “Rest & Heal 8 Hours”
• Nipping into the competitor’s shop
• Teleport away (for example, going to the Castle Ironfist arena and back)
• Load another area (such as Temple of Baa) and return

Works, but is clunky and expensive at the start of the game:

• “Raise the flag and hoist the sail!” twice

Is there a more convenient rule of thumb for when vendors restock?