I've done approximately 20 days with Wiley…and I did my reading in 5-hour stints if I didn't work that day (weekends etc.). Most of the books took about 4 5-hour reading days, though I think FAR took 6 if I remember right, then I'd do a couple weekends of MCQs and be ready for final review week.
I took AUD less than a month after FAR, and I took a week off in between, so that would've been about 20 days for it. REG was about 20 days of study, too – officially was studying it for more like a month, but started a new job at the start of that month, so for the first week or two of the month, was really just working on the new job and didn't do much at all with REG, so basically 20 days of study. FAR I did more like 6 weeks for, and BEC was my first so I studied off-and-on through tax season, then studied for 3-4 weeks after tax season before taking it, cause I was too scared to try anything short with it. 🙂
I wouldn't generally recommend the 20 day route, but it is possible. And…Wiley's books surely are not interesting, but they are thorough. If you can stomach them, they'll teach you everything you need to know. Whether you can learn it from their dry style is another question, but they'll teach it at least.