There's no restriction on the frequency of visitors visas per se. In other words, there's no restriction on the "window," as you say. The major restriction is on the amount of time actually spent in the UK: such a visa cannot be used to establish oneself there. Your girlfriend is very distant indeed from running afoul of that restriction, since she has spent only about three weeks in the UK.
There is nothing in the rules prohibiting six such visits over the course of the year, and there is nothing preventing her from applying for a new visa and using it just as she has used the one she now has.
Problems may arise, however, because of her change in circumstances. That is, she is finishing her studies, so her ties to places outside the UK are weakening. She may therefore have more difficulty convincing a visa officer that she plans to leave the UK after a short visit, despite the fact that she has complied with this requirement in the past.
If she will be moving to and establishing herself in a new place, for example, if she has a nice job lined up somewhere, this is far less likely to be a problem. But if she cannot articulate a clear plan for her life that includes establishing herself outside the UK, there is a very real possibility that her visitor's visa may be refused.
In a comment, you asked
Can anyone elaborate on what evidence would be accepted to prove her intentions to return to her home country?
If you haven't already done so, you may want to refer to the guidance. You should read this as much as you can with the point of view of an entry clearance officer evaluating the visa application. Remember that these officers are required by law to start with a presumption that the applicant intends to immigrate to the UK, leaving the burden of proof on the applicant.
The guidance speaks of personal and economic ties. Since your girlfriend does not have a job, the latter will be difficult, but if she leases a place to live when she returns, this might work in her favor. If she plans to live with family, on the other hand, this will be relatively weaker evidence because it is the kind of thing that one can lie about very easily.
Finally, as I mentioned in a comment, if you can afford a lawyer, you will be well advised to avail yourself of one. They have much more familiarity with the practical realities of visa applications than internet amateurs such as myself could have.
