"6.19 Pirates and terrorists have a right to deal as much damage as they see fit before or after demanding money from a trader ship, but killing trader ship before demanding money is not allowed. Attacking traders or demanding cargo is not allowed for cruisers and battleships, unless cruisers and battleship belong to Terrorist ID or Phantom ID owner."
This is the rule. There is nothing that says the pirate has to wait for the demand to be met.
However it seems to be generally agreed upon that the point of a pirate is to collect cash or loot from traders, so a pirate would want to give the trader as much time as possible to pay. Most pirates on the server will stop shooting you if you as a trader stop running and tell them you will now pay.
Hmm, where have I come? Why does it seem so familiar to this one?