I'd be interested to hear from others as well, but the short answer is that it depends upon when, where and how often you travel. And I'm not evading the question. There are a number of business rules at play, such as how many surveys you receive in a year, how many surveys a hotel has received and more.
I'll go ahead and add a shameless plug here too - the hotel reviews submitted via the community are another great way to provide public feedback after every stay The specified item was not found.
I haven't paid much attention to it till you mentioned it, but I rarely see one of the surveys anymore. Maybet it's because I quit bothering to fill them out> Now when I have a really good stay, I let the GM and Marriott corp customer care know about it...same when it's really bad. When It's just ok, I take it in stride.