A reduced voltage of about half the rated voltage is usually chosen. So, R is about the series resistance of relay. In this case, R, the series resistance of relay and C are forming a time constant of about RC/2, which should equal 100...200msec. So, choose C that RC=0.3sec.

See also here http://www.8052.com/forum/read/168162

The advantage of this current saving scheme is not only to save supply current, but also to keep the self heating of relay and by this thermo potentials to a minimum, which can be important with some measuring and high-end audio applications.

Kai