Since data plans are based on a monthly cycle the best way to estimate it would be to figure out how many hours a month you would be chasing (or at least streaming) and what your average bitrate would be.
Then you could use this formula below, where x is number of hours streamed each month and y is your average bitrate in terms of Kbps (kilobits per second), to get how many KB (kilobytes) of data you used:
(x * 3600) * (y / 8) = KB of data
To get MB just divide the result by 1024, to get GB divide that result by 1024 again.
For example if you streamed 50 hours at a bitrate of 500Kbps, you would have used 11,250,000KB of data, or 10986.4MB, or 10.73GB.