Broadband cap

Hi

I was just wondering if you could give me advice on some broadband services. Due to the minimum contract being 12 months in most places, we can't get a no cap service. So it looks like we have to go for o2 as they offer a month to month service or a mobile modem. So, would 15GB a month be a big enough cap for me? I like to download a fair bit of stuff and often stream videos.

Thanks for your help