I feel like i could easily be a polyglot when I look at how little of my time I spent studying languages, yet by my count I already speak 4+ languages - French, English and Spanish fluently (C2+), German advanced intermediate (B2). I can also read sanskrit and pali at an intermediate academic level, and checked out Hungarian and Russian basics, and have taught both English and French to foreign language speakers.
I suspect I'm not average, but by my estimates one can learn a language's basics in about a month, or get to a fairly comfortable intermediate level in about 3 months of 30-60min/day practice. My secret sauce has few ingredients - first, a combination of two methods, the already highly recommended Assimil method, along with Pimsleur tapes, a listen/repeat method. This ensures you create both the visual/written and oral/verbal foundations. After 2-3 weeks of those, I pick a book with a young target audience, about the level of Harry Potter 1 or 2, something that seems incredibly arcane and inaccessible at the time, but for which you already built most of the grammar and sentence structure basics you'll need. Now all that's left is to practice.
I read the book with Google Translate and a pad of paper. I start at page 1 and whenever I see a word I do not recognize it, without looking at the pad I google it and write it down. I finish the page, read down the vocabulary list 1-2 times, read the page aloud once, and start the next page.
First page will feel like it takes half an hour, the second one takes 15 minutes. After dozen pages or 2, you'll already be down to a couple minutes per page, and by the time you finish the book you'll rarely hit a page where you need to look up more than 1-2 words. You're now well on your way to a comfortable intermediate level and prepared for conversational practice, with a decent vocabulary breadth built-in.
Whatever you do, good luck, practicing languages is such great fun!