From the 3 3GB dongle box in front of me:
Send 3000 Outlook/HTML Emails
Surf the Web for 30 hours
Download 15 four minute videos
Download 96 four minute music tracks
The hours are longer than that in reality in my experience. GF has 3GB dongle monthly dongle allowance from Vodafone and she uses at most half of that. Thats from on average at least 3 hours a day use and sometimes much much longer. That's normal browsing (mostly shopping, I suspect) and emailing. If she watches YouTube or streams music or video then it goes through the roof pretty quick. But without those she gets a hell of a lot more than 30 hours.
Although I think some PAYG allowances might be 3GB *or* 30 hours, whichever comes first.