An expert is a man who has made all the mistakes which can be made, in a narrow field.
– Niels Bohr
Quite interestingly graduate studies usually take about 5 years total in order to obtain a PhD. It can sometimes be one year less or one or a few years over (too long is usually not seen as a good sign however). Assuming that this is basically your full time occupation, have you notice that at the end of this time frame, you will have reached about 10000 hours of dedicate training in your field.
Table above provide the total number of hours spent deliberately on an activity depending on the number of hours per day (horizontal) and number of years practicing (vertical) and was taken from The Brave Discussion blog.
I always tell my PhD students before a thesis examination to expect to be treated as an expert by the examiners. In fact, on the specific topic of your thesis, you are suppose to be one of the lead expert. Over the years, I notice that during the examination the panel will quickly get rid of the more basic questions first, followed by a few detailed burning questions specific to the thesis: methodology, analysis and interpretation. If you do well on these questions, than the process almost become a high level scientific exchange between the examiners and the candidate. In that phase, as a candidate, you should not expect the examiners to know before hand the answer to their questions; this is a much more open handed phase. Here the expectation is that you take the lead as a colleague (yeah at that point your PhD diploma is a done deal!) in the field and build upon your expertise to generate hypothesis, ideas. This is the most exciting phase of a PhD thesis examination. This is the stage you want to reach during the examination.
So of course, the 100000 hours figure to become an expert is not a rule but an interesting guideline that fits the timeline stated above. Of course, mileage will vary depending on the activity in which you are seeking to become an expert, talent needed (physical or intellectual) and drive. As I use to tell people who have interest in martial arts, if in your dojo you can get a black bell within a year, you should go somewhere else!