I should really use the word "trade" rather than skill. 300 hours of work is about 1 month of working a full time job, which is generally good enough to get going and have a hang of the basic nuances of what you need to do for such a job, and all the little things that comprise it. You won't be the best but you'll be handy at that stage, enough that you can probably consider yourself self reliant for all intents and purposes. If you cook stuff for 300 hours, or attempt to do a plumbing project, or do gardening for that amount of time, you'll probably be pretty alright at it by that point assuming you're smart in your self instruction. I think 300 hours of writing short pieces with decent critiques should definitely be enough to put you in a better place.

I've also seen reports that 20 Hours is all you really need to master any particular skill. Which makes me think that 300 should be a reasonable number for something like cooking where you probably can be pretty proficient mastering about ~15 different actions.