I don't know of a rule here, but they do dry out and the effect is an increase in ESR and a decrease in C.  

I have taken old, very large value electrolyetics which had effecitvely gone short after years of storage and reformed them by applying rated voltage through an appropriately rated resistor that limited the leakage current to 25-50mA, for a week.  A bit crude but for hobby purposes I gained several more years service from them.

vjekobalas,

In high-power applications, this can be a real danger. I know two cases when electrolytic capacitors have exploded after extended no-voltage periods. One occasion was in the testing shop at an inverter manufacturer when a frequency inverter was put to final test without "forming" the capacitors. In this case the capacitor exploded "forwards" and hit the person that did the test in the stomach, which caused severe damage and tragically ended his life. The other incident was at a paper mill, where one large capacitor exploded during comissioning of a frequency inverter. No-one was hurt, but the inverter cubicle was so "polluted" by aluminum foil that it had to be replaced.

Some inverter manufacturers now have routines in their manuals that describe when (after how many years without voltage) and how (voltage levels and time) the capacitors shall be reformed.

So, there is a real risk involved with leaving electrolytic capacitors without voltage, but the fire/explosion risk seems to be real only with larger units - above 350 V and hundreds of microfarads.