Altho I can surl a little on my dumbphone, I don"t. Will upgrade to a smartphone when the phone breaks. I'll answer for my brother Dave. He used 110 Mb this month so far on his smartphone. TMobile throttles at 500 MB. The reason he uses data at all is that his employer has a firewall or list that prevents access to many websites.
Approximately 1GB per month (which is my limit). The 4G speeds we get in Australia with Telstra is insane (I've seen 80Mbps!) so it's easy to chew through it so I have to be extra careful.
When I used symbian, I could hit a number around 8 gigs a month, but using windows phone I just need around 1 gigs, both without wifi. Why only one gig? Because the damn OS can't download music and videos through browser.