Alright, so this really depends on how much time you commit to answering questions. I've had my account since around June and I'm at 2259 points, there are some weeks were I don't go on at all, but then there are some nights where I rack up around 80 points. So if you devote the time, you'll get there soon.
Well if you can get 500 points a week, and a $20 Amazon voucher costs 4000 points, then it'll take you two months to get that. But few people can keep up 500 points for two straight months, so perhaps 4-6 months is more realistic.