Is there any average waiting time for an asylum appeal?

Detained cases are supposed to be prioritized and so should not take as long as nondetained cases. If you are appealing you should have an attorney representing you before the BIA. If not, you need to be represented as an appeal is very complex and there are many procedural considerations you need to be aware of. In addition, if you are represented your attorney can advise you regarding time frame. If your brother has been detained for 18 months this is very troubling and you need counsel ASAP to determine what, if anything, can be done. In addition sometimes cases are remanded (sent back) from the BIA to the Immigration Courts and then if the case is relitigated can go up again. Your post does not contain sufficient info to determine why exactly there has been such a long delay. Get help now in a private consultation from an attorney with extensive experience with appeals. Good luck.

Appeals from detained persons are given priority. The wait is usually about 3-4 months. Please see Asylum: Winning Your Case below:

I agree with my colleagues. General appeal processing time for detained individuals is 4 to 8 months. There is no regulatory time limit. Contact your US immigration attorney for assistance.