By law goats have to be tagged by the time they are 6 months old, but unless you are taking her out for a show or selling her (and no inspection looming) I would alo leave it as long as possible... Their ears do grow quite a bit and it is difficult to judge how much space to leave at the end of the tag (if you are using the loop ones that is).
If she is an older goat that has come onto your holding without a tag and you don't know her original set of numbers (from her holding of birth), red tags with your flock and individual number are the ones needed.