A junior lawyer recently explained to me her dilemma. She wanted to have a successful career, to be well-thought of and become a partner at her large international firm, but she did not want to do all that at the expense of her other interests and her mental health.

The offer on the table at her stage of career feels like a binary one: Devote your life to your work, with long hours in the office and little time for anything else, or choose another path. The other path could be working as an in-house lawyer or moving to a smaller firm—neither of which guarantee less stress or fewer hours—or something else entirely.

She is typical of many millennials, who are increasingly disillusioned with the prospect of working brutal hours in order to make a lot of money further down the line. Can they have a successful career without becoming a partner? If they do make the partner rank but their private life falls apart, can that be called a success?

Some define success as becoming the best possible version of yourself. The problem is that being the best lawyer you can be might clash with your ability to be the best parent, which might also conflict with an aim to be the best artist or to have the best social life and so on. There are only so many hours you can devote to each one.

I decided to ask a range of the industry's top lawyers for their views on this tension.

"Success is achieving what makes you and those you love happy. Success is therefore different for different people," explains Nigel Boardman, the Slaughter and May stalwart who is probably Europe's most successful mergers and acquisitions lawyer in a generation. "For some, success, and therefore happiness, is dependent on winning the 100 meters at the Olympics; for others, it is participating in the three-legged race at your child's sports day."

Boardman believes it is possible to successfully navigate a career, hobbies and family life. But others believe there are always sacrifices.

"It's not necessarily impossible to have everything, but it's certainly highly unusual," says one U.K. partner.

One practice head admits that he works too hard and doesn't have many outside interests, while the head of a U.S. firm in London describes how he eventually had to give up one of his favorite hobbies because his work commitments were too time-consuming. Should those be regarded as failures or successes, given that they both reached top ranks in their profession?

Neither, according to James Roome, London senior partner at Akin Gump Strauss Hauer & Feld.

"It is, of course, a wonderful thing to have a passion outside of one's career, but I don't think the absence of other interests should be seen as a measure of success," he says. "The alternative would reflect the false dichotomy of the work/life balance, as if work were not a legitimate part of one's life. In my opinion, one is exceptionally lucky if work—which takes up a huge proportion of everyone's waking hours—is an important and fulfilling part of one's life."

Where this leaves an aspiring young lawyer is unclear. There is no way around the long and intense working conditions. For most successful lawyers, work occupies such a large portion of their time that they can't fit much else into their lives—and, crucially, they are happy with that.

But there is also an important caveat, according to one U.K. partner: "Clients are not only interested in the legal advice they receive but also the lawyers who deliver that advice. It's not easy to be engaging if you've got nothing to talk about beyond the law."

And perhaps that is the most encouraging answer the conflicted junior lawyer could have hoped for.