Any shopkeeper can (as long as they avoid things like racial discrimination) refuse entry to who they like and when they like, without even needing to have any reason whatsoever.
TK Maxx can therefore ban whoever they like (or, rather, who they don't like) for as long as they like. Your friend's ban lasts until the store decides to remove it. It's unlikely that they'd remove the ban without being asked to do so. It's also unlikely that they'd remove it within less than, say, 5 years. However, if the store want to keep the ban in place for, say, 10 years or even for life (which they may well do) they're fully entitled to do so.
Your friend may well be lucky that she didn't choose to shoplift in one of the many towns where the shopkeepers have a 'banned from one, banned from all' policy. If so, she would have had to travel to a different town to do her shopping (or shoplifting?), possibly for the rest of her life.
Chris