I have used a lot of teas in my beers and in fact have brewed a Rooibos Red Ale a few times that turned out pretty well... In that beer I actually added an herbal blend including Rooibos at 15 minutes in the boil, however I do notice that you tend to get more bitter herbal characteristics. I also do a Wood Betony Amber that I steep the herbal tea for 15 minutes and then add at 5 minutes or flameout and I think you get more of a smooth flavor profile. I use this method on my Green Tea Blonde and Mountainside Amber (has Pu'er tea in it) and like it better. Learning more about green tea and how it is traditionally made with such specific brew temperatures and times, really made me understand that teas don't need as much time to steep as we usually leave them. However, where a black tea may only need 2-4 minutes, rooibos needs a little more time like 5-10 minutes and the water should be hot and boiling prior to steeping. So, in your case, I would probably go with a 10 minute steep and add it at flameout.
What kind of beer are you thinking about brewing?
