10 years of work to get to legendary skill, assuming average skill learning rates and no artifacts made, sounds reasonable...except for a couple things.
1. Alright, 10 years of work later and my dwarf is a...Great Mason. Wha? Ah, the idiot was pulling levers/dragging animals to pastures/partying/using the Broker Lifesystem Chart/hauling rocks to the workshop from across the map most of the time he wasn't eating/drinking/sleeping/on break. Dwarves aren't dedicated to their their jobs as much as they should be. Make parties easier to break up (i.e. without undesignating the room, which seems cheap), allow labors like pasturing animals and hauling stuff to the depot and such to be turned off, and make dwarves choose nearby materials more readily, THEN we'll talk.
2. Okay, between tinkering with the speed of learning and how much dwarves work, it takes 10 years to reach Legendary. Assuming a lucky early migrant, that means that you might have a legendary mason, say, in 3-5 years, assuming no artifacts. Congrats! That mason has been working longer than most of my fortresses!
And, anyways, so what? Legendaries make more masterworks? I've had dwarves who weren't even near Master level make masterwork items!So, what do I think needs to happen before this suggestion would be, in my opinion, A Good Idea?
1. Make dwarves more eager to work, or at l;east allow more micromanagement to compensate.
2. Make legendaries mean more than "Oh, I can make my rooms worth a crapton/buy the entire caravan with a bin of goods, but now my wealth is high enough that I need every dorfbuck to feed and protect my population." Let Legendaries craft magic arms and armor and stuff! Have them found worldwide guilds which bring your fortress prestige and power! Whatever! Legendary should be more than the end of skill advancement and a blinky sprite.
3. While we're at it, allow learning more from less. Allow kids, peasants, and so forth to watch friends and family work to gain skills! Allow masons to destroy stone practicing masonry, building up skill faster and cheaper than if they were making plain old stone blocks! As long as training can involve less reagents in exchange for less products and the same amounts of skill XP and time expended, great!