Everyone's requirements are gonna be different, so it's gonna have to be worked out per person. But back before I had ADSL, my wife and I were connecting through my 3G phone, and 512Mb was more than enough for basic browsing, email and the occasional windows update for the two computers. No serious downloading, no youtube, and almost no gaming, etc - too expensive, and line not stable enough.
Now that I've got ADSL, though, things are a bit different - 40Gb local + 5Gb webafrica just ain't quite enough...
My uncle, on the other hand, has an mweb 1gb (and isn't interested in changing - he's been with them for about 700 years or something), and with just him using the line, pretty much goes through that in a month. His usage is mainly emailing photos back and forth to people all over the world.
Then there's another friend of mine who has iBurst 3gb, with four computers in the house connecting to it. They do quite a bit of things like youtube, and he runs a little web based data interchange service for some people on his line, and his usage varies wildly. Some months, he ends the month with 2.5Gb of his cap unused, other months he hits the cap early in the second week already.
My general recommendation to people is to go with something like the webafrica prepaid, buy a gig, see how long it lasts, and do that for a couple of months to see how much you are going to be using on a monthly basis before buying a fixed package of some sort.
