In which physical unit is 'clock drift' measured?
How you measure drift depends what effects you're modelling.

Crystal oscillators tend to specify fractional deviation of the frequency, usually in "ppm" or parts per million. They often specify (or give typical values for) the drift due to temperature and due to aging. For example a drift of 25 ppm per year, means that after a year the frequency might be anywhere from 0.999975 to 1.000025 of what it was initially. 

Communications systems often specify clock wander in terms of unit intervals (UI). Wander measures clock drift over intervals much larger than the baud interval. For example, the clock drift after several milliseconds or longer, for a 1 Gb/s system. A wander of one UI means the clock phase has shifted by one full baud interval. The value specified might be the rms of a statistical variation or it might be a maximum limit.