One of the sites I watch regularly is lifehacker.com, which is where a number of the small anecdotal posts on this site come from.

They recently posted a link to an article from Elizabeth Grace Saunders at the Harvard Business Review that explains a formula for balancing your time and ensuring you don’t over-commit:

(External Expectations) + (Internal Expectations) ≤ 24 hours — (Self-Care)

Those variables don’t make sense in that form, but they each break down into smaller and more logical things. Start with finding out what your Self-Care number is. This means adding up the time you need for sleep, eating, and personal tasks, such as grooming. Now add up your External Expectations like work, commute, pets, and relationships. Lastly, add up your Internal Expectations like hobbies, side projects, and personal time. Here’s an example of what mine might look like:

• (8 for work + 1 for house and pet duties) + (2 for family + 1 for reading + 1 for side projects) ≤ 24 hours – (8 for sleep + 1 for eating + 1 for personal grooming)
  
• (9) + (4) ≤ 24 hours – (10)
  
• (13) ≤ (14)

When you add it all up, is the left side of your equation less than or equal to the right side? If not, you’re over-committing yourself, and that can lead to burnout really fast.

Source: A Formula to Stop You from Overcommitting Your Time | Harvard Business Review