The sentence
She'll sit talking to herself for hours.
may express typical behavior. Indeed, M. Swan in his Practical English Usage (629.7) says: "We can use will to talk about typical behavior" (habits and characteristics).
But we can also use the present simple to express one. For instance:
She sits talking to herself for hours.
What's the difference between expressing of typical behavior via will and the present simple?
The Cambridge Grammar Of The English Language (p194) lists this function of will as expressing "propensity", and gives the example: He will lie in bed all day, reading trashy novels.
The CGEL goes on to explain:
Here we are concerned with characteristic or habitual behavior ... A simple present could be substituted with little effect. ... Strong stress on the auxiliary conveys the speaker's emotive response to the situation - usually exasperation, disapproval, resignation, or the like: He WILL pour tea-leaves down the sink.
So, both She'll sit talking to herself for hours and She sits talking to herself for hours will likely be interpreted as simple statements of fact, whereas She will sit talking to herself for hours will be taken as conveying the speaker's (usually negative) evaluation of that fact.
The "will" usage has also been mentioned in Longman Dictionary of Contemporary English as "annoying habit":
used to describe someone’s habits, especially when you think they are annoying, e.g.
Trish will keep asking damn silly questions.
