I've burnt out twice, it ain't pretty and once you've experienced burnout it opens doors in your brain that are difficult and time consuming to close again. So you're best off making sure those doors remain closed by taking care of yourself :)
For the first five or so years of my software company I worked like a dog on meth. It wasn't smart working, it was sheer brute force and ignorance that got the business off the ground. Come Christmas every year I would be a shell of myself. Christmas was the only time I really stepped back from work, and I inevitably got ill most years as my body recovered from the sustained beating I'd given it that year.
My first burn out was 2011. The recession had kicked in, clients were going away or shrinking their spend with us in droves. To compound matters my personal finances were intricately linked to the businesses success or failure due to the personal guarantees I'd signed on loans and the office space. I started to suffer panic attacks and general anxiety. I tried to self medicate both positively (exercise) and negatively (drugs), but ultimately hit the deck hard.
The noise in my brain caused by the anxiety and stress had my brain at 100% CPU all the time, leaving nothing for work or, well, anything. It took about three months before I could go back to work in any normal sense of the word.
Fast forward to last year (2015) around Christmas time. Another very busy year but this time business is brisk and so I was rolling around in our moderate successes. Again, December came, and I hit the deck. This time I had a wicked health scare to boot. It took about four to five months, so slightly longer, to recover from this one.
Fool me once and all that. I've now restructured the business so that it's almost impossible for me to work myself to death. I appointed a managing director to take care of the day to day stuff that was burning me out, leaving me to focus on strategy and leadership - the things I'm good at.
It's ironic that I called my business Atlas - Clever Software because he's the Greek God condemned to hold up the sky for eternity. I condemned myself to holding the heavy weight of a very complex and difficult to scale business. I'm very lucky to not have long term physical or mental issues as a result of the stress I needlessly endured. I know other founders who are not so lucky.
It's due to this backstory of mine that I have an deep dislike of the 'hustle' culture. It's alright to bust your backside to get a business off of the ground, but at all times you owe it to yourself and those who love you to put your health and wellbeing first. You've got one body but there's millions of business/work opportunities out there that you don't have to kill yourself to make a success of.