urine enemas - how often, old or fresh

I've been searching the forum for info and haven't yet learned how often it is considered safe to do an enema. Armstrong spoke against them during the fast, saying that the bowels will move when they have recovered the health to do so. Other sources speak for enemas.

Is old, pungent urine safe as an enema? I have read that it is, yet wouldn't the ammonia be absorbed, rather than gassing off as it does when massaged in?

Thanks