NPC price

so I learn that NPC price item following a supply-demand mechanism, it the more you buy a certain item in consecutive time, the more price increase

How long do I have to wait till the price stabilze?

That only applies to guild merchants. Unfortunately I don't know the answer to your question, but I'm sure someone else does, I just wanted to clarify that part.

really? only on guild merchant?

Im asking it because Im doing Fame leveling up using millioncorn and la theine cabbage, and the only reliable way to get it is from Cullinary Guild in WIndhurst

http://ffxi.gamerescape.com/wiki/Millioncorn

Millioncorn can be bought in whitegate too.

I haven't play that long and I only know the three big cities/polis. I haven't adventuring much really
too much flow of information lol