There are not waves of attackers, per se. Armies move around on the world map, and cannot initiate an attack unless, 1 - they are at the location, and 2 - the people they are attacking are not already in a battle. Thus, if you pay someone off, you can go right offline, since there is virtually no chance of another army being right there. I "believe" that there is a timer too, so that one player cannot repeatedly attack and drain you.
"Yes... I am writing a book. So far I have the page numbers." - unknown author.