Short version - It's not about how many months, but how many hours in those months.
Typically
Working with video or a book - Instruciton hours, practice hours, lab hours
Linux competency - equivalent 3 credits (if 1 year at college = 30 credits)
100 hours instruction / 20 hours lab / 100 hours homework.
Linux proficeincy - dquivalend 6 credits (30/year)
200 hours instruciton / 40 hours lab / 200 hours hoomework.
about the same as a year long or intense term course.
Linux expertise - equivalent of 9-12 credits (30/year)
350 hours instruction / 100 hours lab / 300 hours homework
approximately 750 hour
3 months - 90 days - 2160 hours - So if you devote 8 hours a day 7 days a week you'd get there. There are ways to accelerate this, but this is roughly equivalent. If you can deovete this kind of intensity, you should be able to get there. BUT THAT DOESN'T INCLUDE 4 HOURS OF checking sports scores while you do it.
/aj