Before Marriott even knew what an Ambassador was, it generally took a couple of weeks to get one assigned. When I first made the level in 2014 I pushed to get the status before a big vacation and had to make a few calls since it was not an immediate thing.

And as I've mentioned before not all Ambassadors are equal, some clearly love their jobs and taking care of us, for others we are an after thought. Just remember even if you have an assigned Ambassador you can always call the generic Ambassador number and they will help you, I've gotten some great help when I had to call outside of my Ambassador's normal working hours.