I have learned a lot about this from Benedictine nuns and monks. According to Rule of Saint Benedict time should be equally divided to work, prayer and relaxation. A bell is rang to remind everyone when its time to begin the next segment of the day. Benedictine monastic "enterprise" managed to thrive this way 1500 years with happy nuns and monks.

There is always room for flexibility, particularly when we get inspired to perform a certain task and do not wish to get interrupted. In our own business, especially if this is the right business for us, we tend to go into flow during our work. In this state we lose track of time and we do not get tired. We can allow ourselves to go into flow from time to time in some of our projects.
We may also wish to increase our working hours, for example when our business has just entered the stage of rapid growth.
However we can be much more efficient and provide better quality service if we limit our work to a reasonable number of hours.

In an average day the best option is to plan not only work but also leisure activities, keep photos of your loved ones on your desk and set up reminders in your watch or phone. This will be like imitating life regulated by a Benedictine bell, which can be helpful in fully enjoying all the aspects of your life.

Feel free to contact me if you have more questions. 
I'm an Industrial Psychologist specialized in personal happiness and life satisfaction.
I can also can help you find the right business idea which will bring you the flow experience and make it easier for you to become successful.

1. Are your tasks clearly defined? Lack of clarity can drop the enthusiasm level.

2. Are you excited about the tasks? See above.

3. Are you clear on the reward for completing the tasks?

4. Is there simply a rhythm your body and mind WANT to work in, that you'd likely be best to listen to? I'm a "Grind It Out" guy and work on things to death once I start, but some people really like Pomodoro, for instance.

Also, we all have our "Golden Hours," a topic I was quoted in Inc. Magazine about...times during the day when we're best suited to do certain work. In my case, I am best at "grunt" work in the morning...non-creative, bang-it-out type stuff. And I get tired from about 3:30PM - 6:30PM so I try not to book anything but podcast appearances in there (which instantly put me on Performer Mode.) Then from about 7:00PM to 1:00AM I am creative, so I schedule those tasks then. Respecting these preferences is important to my success. Can I do creative copywriting in the middle of the day? Sure. But the end result will probably not be as inspired as it could be.

As Dan Ariely said, we don't know our preferences that well. If you learn yourself, you can use that machine to its best advantage.

Here is a methodology that might help.

1. Understand that at some periods of the year there will be NO worklife balance. And that's ok. Try to minimize them and prepare for them.

2. What do you need these extra 10 hours per week for? Gym? Sleep? Family? Hobbies? Relaxation? Where would you start first?

3. Make a concrete plan on how to win back these 10 hours. Say No to some meetings. And learn to Say No in general, so that you make room for your Yes.

4. Create your boundaries and be accountable for them. E.g., no meetings or phonecalls after 18:00. No replies at emails during the weekends. Train others to respect them. Respect them yourself.

5. Work is also part of life. Work your way through feeling better at work.

People talk about time management all the time. But I think, energy management is just as important as time management for working smarter.

I wrote a post on the topic of energy management a while back which you may like. Check it out here: https://medium.com/the-mission/the-ultimate-guide-to-managing-your-energy-on-purpose-so-you-can-thrive-at-work-and-life-6932a871f504

Think about what you are scheduling and how you manage interruptions.

People often schedule their work commitments and not personal ones. Whatever your idea of "break time" (a hobby, time with family or friends, workout, rest, etc): schedule that into your calendar as if it were a business meeting. Making it a regular time will help, for example every Tuesday and Thursday at the same time, etc.

Interruptions tend to be where most time ends up being lost, as they throw everything else off schedule.
It is ok to not pick up the telephone at that exact moment, or to not have an open door policy. Tell people to schedule their ten minutes to speak with you and when ten minutes are up, end the meeting. If they need more time they will learn to schedule more time.

It depends on the intent of the question.

If you are talking about having breaks during your working hours, then I find the Pomodoro method a great way to structure my day. I set to between 40 and 50 minutes and then take the break between to walk around for a few minutes, have a coffee or chat to someone.

If you are talking about work flowing over into your home life. In this one I believe we all have to find the right solution for ourselves.

I have a 3 year old, therefore no work is allowed to disrupt the time between getting home and his bedtime. But, I will often work once he is in bed, emails and sometimes returning calls.

On holiday I will work for an hour each day.

But, I also gain flexibility. If I want to take the afternoon off I can, if I want to go to the theme park I can arrange during the week when it is not so busy. If I want to go shopping I do during the week and always avoid shopping at busy weekend.

Learning to know when it is enough is the hardest part of working for yourself.

Do you have goals? Have you prioritized your goals?

One of the things I have done that I have found useful for me is to shut out distractions, create time and a place to focus. The biggest mistake I see startups make is the lack of focus.

As for breaks, schedule breaks into your day and set reminders on your calendar, phone, or Apple watch.

1. Have downtime and breaks.

2. Don't focus on work-life balance. You only have one life. So, for example, don't be afraid to take off a Wed. afternoon, or work a bit on Saturday.