I get them once every week or two. Seems like it's a few thousand wagered triggers it or maybe they give everyone with some sort of action in a given period the bonus. I have even received it when my account wasn't funded due to recent cash out.
to me it doesnt seem random, when i play there i get the bonus every week, when i dont play i dont get it
its definitly based on the turnover you generate, but i dont know exactly figures, the good thing is they give it to you win or lose, you could be up big and still get it. also, the amounts vary depending on your turnover, I personally have gotten 5/10/15 euro bonuses from them, the more i bet the bigger the bonus, i wonder if they go even higher than 20 if the player is big enough