I look at it as how many forts it took to learn.
My first fort I had no fucking clue what anything meant and what you were supposed to do. I promptly abandoned didnt come back for a week or so and decided to take a look at the wiki.
My second fort I learned to dig out a fort, set up workshops, fish, that pumps pump liquids to the level of the pump, i.e. how not to flood your fortress when trying to make a well. I eventually said fuck it and purposely drowned my dwarves.
My third fort which I just remembered now after trying to remember all this was a pretty cool fort. I think I learned to farm on that one and set up traps, but decided to drown my dwarves again after discovering obsidian casting and went looking for a tall map.
And my fourth fort I learn how to set up a military, provide housing, general food production, get magma forges up and running, how to build pump stacks, perform controled collapses, and little things in general. I planned to carve out a giant hole in the side of a mountain and magma cast a nearly 100 z-level tower but I got bored with it, especially since I learned about HFS during this fort and that megebeasts didnt always survive. There was no HFS in this fort and were no living megabeasts so I just stopped playing it.
By that time I pretty much knew how to do everything, even if I havent done it yet. The wiki was the biggest help. So really it took me 3 forts to figure everything out. I'm not sure how long the timeframe was for all this.