I did 40 hrs/week day job and ~10-20 hrs/week consulting for about a year. The income increase was substantial but so was the impact on my free time. All things considered I'd probably still be doing it if I didn't become a dad.
My advice would be to make sure you have the time, be wary of long term commitments in case you hate it, and give it a shot if possible. Make that hay while the sun shines!
One other caveat: I was lucky enough to get a single stable contracting gig covering all of those hours. Had I needed to spend more time finding clients I may not have had such a positive experience.
I work 35 hrs/week (7 a day) so 10-20 hrs/week of freelancing should be feasible.
I'm well aware that it means sacrificing a lot (if not all) of my free time but as I said in another reply, I do have a lot of free time since I don't have any hard obligations besides my day job. Also I'm aware that one day I might have less free time, that's why I'd like to try this out while I still can.
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