Great idea if you want to stay in Japan.

Otherwise it'll 100% depend on what results you're able to produce, and where you publish. The threshold for top publications from UTokyo is higher than top-class universities in the US or UK, so the same research is harder to publish as well than if you were at Harvard. It's not uncommon to get scooped by top western institutions before finishing a project, as well.

The automatic progression is great for getting a degree in a set period of time, but not as great if you have a very successful project and need a few more years to get that Nature paper. Also, unless you're on scholarship, you have to pay for the degree.

As such, it depends on your lab. Hours wise, completely dependent on your lab. The slavedriving professors probably won't work with you or won't hold you to the same expectations. Even the best labs hours-wise will find people staying until late at night just because that's the culture, though there's no requirement.

Mentorship isn't really a concept in Japan (not that it's great abroad either). You're expected to read papers and teach yourself what you need to know, and at a stretch ask the mid level profs in your lab if you get lost. There's very little in the way of teaching in most labs.