Target age range, they can only think two days ahead. Also consider that in a two-day makeover you can explain it away why nobody ever has a change of wardrobe or talks to other people or has anything else going on.
Is this character like school age? If so, then "months" of training would have to explain how that fit in with schooling, and perhaps that wouldn't work with the rest of the plot development.
I get the impression she is meant to be a Teenager, likely 16. Judging by her looks I'd say young adult, but... this is Disney we are talking about. (Though it does help that she is actually like... 60 years old)
--Though she doesn't go to school... Not even highschool and has no tutor... While her sister goes to elementary (In a... fantasy medieval-esk setting)... So she could be a young adult.
As for "Explain away lack of wardrobe or doing anything else"
Man this is a children's cartoon. You know people only have one set of clothes, and that they don't actually do anything when the main character isn't around
-I kid, she has two sets of clothes
I mean do you think the Crystal Gems had any real issues after Steven became a Crystal Gem but before he went on missions? No! Because he wasn't involved...
In fact... all their missions go a lot more smoothly when he isn't involved.
In fact... Connie's training seemed to go a lot more roughly the moment Steven was involved...