That depends. It took my daughter just a few minutes. I waited for two hours after I attempted activation with AT&T, with no luck. I called again and it took about two minutes-the CR who helped me said that activation should take about ten minutes. My old phone deactivated that quickly but my new iPhone 4 still did not work. I'd wait about half an hour and call AT&T again if it does not activate within that time. With other phones I have waited an hour or more. I thought ten minutes is not much time and overly optimistic-but that was all it took for my daughter. So give it the half hour or so.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.