On normal/normal, I started as Duke of Aquitaine. By 1200, i've conquered most of Europe. Only Finland, Rus, Denmark (which includes parts of Poland), the Baltics, the Empire of Byzantium, and the Seljuk Turks are outside my realm. I hold 30 (if not more) King titles.
We'll see what the mongols do to me, I haven't played CK that far in ages... but anyway, this was achieved in 130 years. The Game gives you almost 400 years. Without an external force like the mongols the whole map could easily be conquered in less than 400 years.
Problem is, I can tell you that once you get a few historical nations under your rule, actually ruling them starts to get progressively more tedious. Vassals attack you, each other, and declare themselves independent. Each ruler gets realm duress once or twice during their reign (more declarations of independence and vassal dow's). The repetitive events and micromanagement of vassals get tedious.
For me, succession has also been a problem, i've just switched to elective law for the second time so I can have bastards inherit me, as i've run out of legitimate heirs (the previous time the only character left in my dynasty was the ruler's bastard uncles son, and thus excluded from inheritance under anything else than elective law, this time my current character only has two bastard sons despite assassinating two wives in an attempt to marry a more fertile one).
I'm still planning on playing this to the end, and looking forward to the mongols giving me some real challenge. As it stands, I doubt i'll play a "conquer everything" game very soon after this, if ever again though. It's just not fun after a certain point is reached (when you're powerful enough to conquer any other realm if you want to). This is just not the most entertaining way to play CK in the long run, although it can be done.
That's not to say you shouldn't try conquering everything, go for it. It's worth seeing what it's like to rule (or attempt to rule) all of Christendom/Europe/the known world. I'm just saying, be prepared.
