Adjustment time depends on the horse. Some horses take longer then others so saying 1 week, two weeks, or 20 days is arbitrary.
I think a lot has to do with the experience of the horse, and their personality. If it has traveled a lot done shows, and rides away from home, ect I would think the adjustment period would be shorter then one who has been in the same place its whole life and is then uprooted.
I have one horse who travels well and adjusts to new surroundings, his motto seems to be home is wherever there is food. My other one is sensitive by nature, hates traveling and has a more difficult time adjusting to anything. Both extremes on the spectrum of adjustment.
I think when moving a horse to a new environment it is important to keep things as normal as possible. Keep the same feed, keep the same feeding schedule, if you normally ride then keep riding, exercise is good. In any event you will be the constant for your horse.
Most horses will not refuse all food and stop eating. What I fear is some tend to stop drinking, new water for some horses is difficult to adjust to. They colic because they are not drinking enough, and they get an impaction because they are eating, but not drinking.
Getting them to drink can be a challenge, the old adage you can lead a horse to water but you can't make them drink. But you can put some extra salt in the food which helps to encourage drinking, you can feed wet feed like soaked beet pulp and hay cubes, so there are ways to get water into them.
Tell your know it all "crazy horse women", thanks for her opinion. Talk to the owner who actually knows the horse and ask how they think the horse will adjust to new situations.