I think there are 2 main reasons for vow renewal.  A) Because your first time was in the court house and now you can afford a formal ceremony.  B) You want to celebrate and strengthen your marriage via renewing that "in love" feeling.

Either way, I don't think there is any etiquette as to an appropriate time.

Talk to your spouse, or maybe surprise him/her with the "wedding" you wish you had (either elaborate or eloping)! 

Would the same go if you are considered comman law marriage, and still want to have a court house marry, then go ahead and have the big day a couple of years later? We already have a house payment, and car, and baby, so we are not spending any money anytime soon.