I lasted 1 month on my iPhone 4 without a data plan. WiFi support just wasn't ubiquitous enough. I've been on a data plan ever since, ranging from about 1 GB to 6 GB per month. I'm on a 2 GB plan now, which is sufficient for my current needs.
My wife is on a 450 MB plan. She didn't have a data plan with her dumb-phones for the longest time, but now that she is on a smartphone she sees the need. Her data needs are not heavy, but there is still a need. I don't think she'd be able to function well with less than 250 MB per month.