Wondering how much data a 3G video call (say 20 minutes) would use, and which data plan level (monthly GB allowance) would be suitable considering daily calling? From viewing past threads, it seems that "unlimited" is not truly unlimited.

Which application will you be using?

Typically they have some variability in audio/video codecs and associated bit-rate, and a quick search can typically yield this detail for your app.

With something like Skye you might consume 600 Kilo-bits per second (for both audio and video, and for both upstream and downstream). If you're good at math you can figure out how many bits a 20 minute call would require, then convert that to Bytes.

It would be on FaceTime.

According to one analyst's estimate, those on a data plan that offers a single gigabyte per month could use up the allotment by making just one five-minute call a day on FaceTime.

http://online.wsj.com/news/articles/SB10001424052702303703004577476741029162690

From which I infer you may be able to get 150 minutes of video-calling per month?

You could probably find more experiences with a search of some Apple-specific forums/sites?

Thanks for that estimate. On the Apple site I found out that the data rate is 4-6MB/min so that's about 100MB / 20mins (300MB/hr). It suggests a plan with a monthly allowance of 3GB+ would be necessary.