If you have other sources of income, or live with someone who helps with the bills, you cannot truly understand the strain of making an income via freelance writing. You can write part-time and be fine. Folks like me who are single and rely only on ourselves have to make a huge time commitment.
The amount of time I personally spend working every week is monstrous. Add in full-time nursing school, being a caretaker for two people and having no-so-great health and it takes its toll. However, the end benefit of financial security, as well as financial independence, is well worth the roll of the dice.
For me, just getting by is just not good enough. I also enjoy having savings and the ability to have a few non-necessities from time to time. I am willing to put in 80 to 100 hours a week to have a comfortable life. Plus, I love what I do so most of it does not feel like work.
Now, I cannot tell you how much time you personally need to spend writing, as all of our situations are different and unique. However, I can tell you that being a freelance writer is exactly like owning a business. Businesses take time commitment and sacrifice. Are you willing to do each of these? If not, then there is a high chance that a career in freelance writing is not for you.
If you are not the sole bill payer, you may be able to get away with a lot less hours than I put in. If you are like me and are single and completely self-reliant, then the time commitment will likely be very big. Most will never have to work the hours I do because most people do not have the bills that I do (multiple monthly medical expenses and I am paying for nursing school completely out of pocket plus all regular adult bills).
I will say this: never kill yourself by working yourself into the ground, but never think that you can skate by by writing whenever the mood strikes. You need to commit a certain amount of time each week based on your goals and stick to it. This is a job afterall, and should be treated as such.