I saw in wikia

 

"Similar to the Barrows equipment, combat robes will degrade over time, starting with 100 charges and becoming fully degraded after 100 uses. "

 

so does this meen they become useless after 100 spell casts or after 100 time using the effect of the robe (While wearing a complete set of combat robes, there is a chance that the robes will supply any mind, chaos, death or blood runes used when casting a combat spell.)

After 100 times using the effects of the robe you will have to recharge them again with F.O.G tokens.