I can't answer how long she will teethe, but I can tell you we used the big fat plastic straws as teething aid for our kitties. They love to chew on them and they are safe. When one would get chewed up a bit, we'd just garbage it and throw down another one or two.
When Charlie was about that age (till 7-8 months old) he used to nibble on the dog's rawhide bones. Still does once in awhile Maybe you can get some chicken/beef flavored small rawhide bones to give him. They just nibble on the edges - they don't eat it