Easily more than 20. That's a very conservative poll right there. I think any site where people are regular enough to vote on such a poll will likely visit at least 3 times a day, ie: once in the morning, once in the afternoon and once at night, and consider that light lurking, which is over your 20 a week. If people make a post they'll likely visit easily 10 times in one day, just to see if anyone's taken interest in what they've said, even if they don't have a desire to make any further posts. If you're in a debate with someone, jeez, you could do the whole 20 in one day easily.