Beware, it has a subtle catch:

    (1 TB) / (300 (MB / month)) = 291.271111 years

But if you think you're dealing with decimal Bytes, you're wrong - e.g, putting it in GB and taking the units out:

    1000 / (0.3 / month) = 277.777778 years

(http://en.wikipedia.org/wiki/Gibibyte - Google is using TiB/GiB/MiB and writing up the wrong unit)