I tested it a couple of times last night, and as it takes 5mins off time, is best to take at 6-7 or 8 mins into time elapsed. And can only be taken once per visit.
I have verified with staff and what I got confirmed by Essie now is that, weirdly and complicated enough as it may sound, yes you have to drink it WHILE *ON* Ancestral Land with BUFF on, and then it will affect you once kicked OFF the Ancestral Land with your DEBUFF, which should be 25 minutes instead of 30 :) Makes sens? The other way around would have been so much better haha!! XD