I left O2 a few months ago, had 1GB on that which was enough but mainly because O2 signal was so crap round here it was barely usable. Now on better but not great connection with Vodaphone and a 2GB limit which has been fine so far too, currently have ~800MB left with 8 days to go, though we spent a long weekend in France so used it more there.
I use a lot of twitter, bit of facebook, some snapchat, what's app and general on the move browsing plus maps a reasonable amount when away from the area I know.
The better the connection, the more I tend to use it...