I've read that the Eveready's will last ~150hrs with the CM300's. I bought a new pair after a year (~85hrs), and carry the "old" ones as backups. Just be careful that you dont leave them on in the mic body. Happened to me once. I usually take them out of the mic, but security was hurrying me along at a show, so I didnt take them out and left one on for 2 days in my gear bag.
Jim