I think the answer to this question varies from person to person, and depends greatly on the commitments you already have, and the things you value investing time in.

Back when I was at university, my honours supervisor was of the opinion that I should be sleeping in our lab, and doing as much work as possible as often as possible. At the time, I'm rather glad that I ignored this advice because your social life tends to suffer enough anyway, and I had friends, a motorcycle, and a girlfriend that I wanted to spend time with also. In the end I still graduated very successfully, despite what my supervisor's expectations were.

After I graduated, I decided academia wasn't my thing until I knew 100% what I wanted to study, and eventually I slipped into spending a lot of time playing video games and watching TV, and this increasingly became uncomfortable for me, so over the last year or two I've been working on spending more time doing things I find more valuable, in a more structured and versatile way that enables me to quantifiably make progress on the things I care about, while still allowing time for spontaneity, etc.

My current system ultimately comes down to a daily set of checkboxes that reflect the things I find important at present:

- projects (any progress)
- reading (30 minutes or 1 chapter)
- exercise (walking/running/pushups/swimming)
- other (catch-all tasks, errands, etc)
- up @7:30
- bed @11:30

These are mostly non-social goals, and my aim is to tick off as many as possible, but I give myself permission not to worry when something else comes up. Christmas is always a write-off, for example. But primarily, it gives me motivation to make time for the things I find important when I am able to, because I too have to work 38 hours a week, a social life that I would like to maintain, a relationship that I'd like to keep, and a smattering of other random things from time to time, and it's not always easy.

So in summary, I guess my answer is to find the balance that works best for you. The people you hear of that seem to work 10 hour days then throw in another 4 hours for side projects (or whatever) have found their balance that likely involves a number of sacrifices and compromises that may not actually suit you, so you should not aim to emulate them, and you can most definitely have a successful career without doing so. I would argue however, that your 'successful career' should be your own decision, and not because you believe "that's what people do". Your definition of 'success' may be very different to what the corporate world wants you to believe is successful. 20 years of 18 hour days working for big names may be one definition of success, but it certainly doesn't have to be yours.

Finally, to answer your question of 'what will an extra 30 minutes a day get me?' - possibly not much if you only look at it in 30 minute chunks. But 30 minutes is a lot of time, when you accrue it over time. Approximately a week and a half per year if I've done my math correctly (or 22, 8 hour work days per year!). That's not an insignificant amount of time, and if you work on something important to you, it can be very rewarding. As far as skill development, you will likely learn more in a challenging environment (for a business, a startup, or with a group of people more experienced than you), but you can definitely do something valuable to you with that time.