@Chaosmancer Because I'm apparently a glutton for punishment...
How many years does it take to become a practicing Wizard? To go from "I literally only know how to read and write" to "I'm a 5th level Wizard," given that's your stated target. Keep in mind, we're talking about people who aren't adventurers, so they aren't delving into murder-holes and sparring with bus-sized lizards with superiority complexes and epic halitosis.
How long does it take?
Well, lets see.
The image of an Apprentice wizard is a youth, looks to be about 12 or 13, and they are 1st level wizards. The images of professor wizards able to cast 7th level spells (so far beyond 5th level casters) look like they are in their 30's. Leveling is exponential to a degree, requiring more and more to get higher.
So, I'd say about 4 to 5 years. Maximum.
Of course, that's just wizards. Warlocks, clerics and paladins will probably be more variable, and likely much faster. Bards will be about the same, maybe slightly faster. Sorcerers depend on when you want to start counting.