When the client is happy and/or they pay you.
Even then, we’ve all seen shameful work that will never do its intended job, which clients have been ‘sold’ (and presumably paid actual money for) by unscrupulous ‘designers’ who have never learned their craft and have no idea what they are doing.
If you really want to learn, in order to make a career and take money from clients for a product, I’d always recommend getting an education. At very least, get an entry level job in the industry and learn that way.
If you want to do it for fun and for yourself, then learn from the best and read everything you can. Don’t just rely on the internet. Read what people like Wally Olins, Alan Fletcher, Massimo Vignelli (to name a few) have to say.
Learn what a logo is, in relation to a brand. A logo is not about a pretty adornment to fill a hole at the top of a website or letterhead. It has a job to do as part of a coherent whole.
Please, please don’t just look at a few Pinterest pages and think you know what you want, then simply add to the increasing cacophony of visual noise out there. Add something to the pot, don’t just water it down. It is definitely not one of those subjects you can learn on a rainy Sunday afternoon and, bingo, you’re a designer. Most of us have either done a degree, or learned on the job, mentored by people who have been doing it for 20+ years, then gone out there and gained our own stripes with at least 5 years’ experience before even thinking about selling our wares to clients as a freelance designer.
What is your goal? Is it something you want to pursue as a career?