We address the topic of the intra-night optical variability of the BL Lac object S5 0716+714. To this purpose a long-term observational campaign was carried out, from 1996 to 2003, which allowed the collection of a very large data set, containing 10675 photometric measurements obtained in 102 nights. The source brightness varied in a range of about 2mag, although the majority of the observations were performed when it was in the range 13.0<R<13.75. Variability time scales were estimated from the rates of magnitude variation, which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027mag/h, corresponding to an e-folding time scale of the flux {tau}F=37.6h. The highest rates of magnitude variation were around 0.10-0.12mag/h and lasted less than 2h. These rates were observed only when the source had an R magnitude <13.4, but this finding cannot be considered significant because of the low statistical occurrence. The distribution of {tau}F has a well-defined modal value at 19h. Assuming the recent estimate of the beaming factor {delta} about 20, we derived a typical size of the emitting region of about (5x10^16^)/(1+z)cm. The possibility of searching for a possible correlation between the mean magnitude variation rate and the long-term changes in the velocity of the superluminal components in the jet is discussed.