This depends on where you are in your life. When I was in my early 20's, I had my main day job, and 4 side jobs - one was my own Freelancing. It gave me little time with my friends, but I was making money and learning new experiences.
You're lucky you have a manager who wants to encourage you to go out on your own as well. I've never ran into one who was excited about me working for myself.
What I would suggest is to try it out on a part time basis first - maybe 3 hours every second day after dinner (you still need your basic 3 meals per day). If you can handle more, slowly increase your workload. Once you feel you've had enough, scale back slightly, and keep that limit. You will need to re-adjust as you go on, but that's part of the challenge and responsibility as a new company owner.
Heed my warning though - working 18 hour days starts to wear on people quickly. As soon as you know you'll have lots of steady work with a steady income, I'd drop my day job and work only as a Freelancer, and not stay with the one company. You don't need the stress of your day job on top of stress of being a new entrepreneur as well.