The read range depends on many factors: the frequency of operation, the size and shape of the tag, the power of the reader, interference from metal objects or other RF devices. In general, low-frequency passive tags are read from a foot or less. High frequency passive tags are read from about three feet and ultra high frequency (UHF) tags are read from 10 to 20 feet. Where longer ranges are needed, semi-passive and active tags use batteries to boost read ranges to 300 feet or more.
As a rule of thumb for Low and High Frequency, if the size of both the reader and tag's antenna are equal, you can take the diameter of the antenna and multiply by 1.4 to give the read range. This will hold true up to around 30cm (12") and then other factors come into the equation.