Facetime data usage?

Depends on the length and type of call.

Video calls will use more than audio only calls.

But consider about 3 to 4MB (megabytes) of data per minute of a video call.

If you are on cellular data, you can go to Settings>Cellular>Cellular data and can see a list of apps and the amount of data they have used since the last time that you reset the counter. If wi-fi, there is no way to measure.