The safest way is to lay it out and extract the parasitc R and C.. If the decoupling cap is large I would be concerned about the R you can get towards the supplies.. a bad (e.g. high ohmic connection) makes your decoupling worthless.. Impossible to give a rough estimation of the values, it's heavily dependant on technology !
In case you are talking about MOS decoupling capacitors, check:
P. Larsson, "Parasitic Resistance in a MOS Transistor used as on-chip decoupling capacitance", IEEE JSSC, pp. 574-576, April 1997.