Not a meaningful concept. 
The only consequence of the excess voltage, is increased operating temperature.
The standard "Songle" relay operating at 90 mA has a coil resistance of 55 Ohms. At 5 V it will dissipate 450 mW; at 6.5 V it will dissipate 770 mW. It will get a little warm if actuated for long periods.
To drop 1.5 V at 90 mA, you need a resistor of about 17 Ohms. The nearest common value is - well either 15 or 18 Ohms would be just perfect! 
Not a good example though. 
470 or 560 would be the nearest common values.