The only definitive rule on this is "You must make a demand, or you may not kill the trader."
IMO if you say, stop, pay a donation, and he stops and opens the trade window, you should then specify a set amount.
@Jinx,
I don't agree with what you've said.
I can tax a trader, say 2 million. He pays, and then he goes and says "filthy corsairs", I'm not going to just let him go now am I?
Likewise, if I tax a trader only 1 million, he pays, is about to leave, then I get some information from a buddy that the trader's a known passenger smuggler, has attacked OPG before, etc. I'm fully in my rights to go, hey you stop, looks like I didn't tax you quite enough.
You do not pay tax because it somehow buys you a magical safe passage, you pay it because if you don't, you die.
Simple as that.
It makes perfect RP sense, and there is no rule against how often you can tax a trader.
