It shouldn't take as long as it does. In fact, it didn't used to. Used to be instant. This combined with people still reporting issues getting their zen stipend will keep me from giving this company anymore money. At least until these issues are resolved.
No method exists for this except to track it yourself. There is absolutely no way either on your account page, or in-game, to track stipends. The C-Store will indicate previously purchased items but will not show when they were purchased.
As far as I know, there is no way to track stipends other than manually. The transaction history only shows Zen bought and transferred. Not having a transaction history detailing when stipends were given is, at best, poor customer service and, at worst, a way to facilitate TRIBBLE over the customer base. For all it does…