I don't remember the specific number, but in my longest playthrough (14 game years) I think it was somewhere in the high 20's. That character hit at least 25, but am sure didn't get to 30. This was roughly during patches 1.0.7 up to beta 1.2.0.
22, keep in mind that the reduction in learning speed for all skills each time you level up make 'what' you skill up first more important then the max level.
Basically the FP and attributes you gain after say level 16 start be much less useful since everything is so much slower to raise.
So I might say 16 is good for looking at a 'build".
I got to 23 tier 6 as an independent king. I had about 25-30% of the map with vassals coming and going, but was overcome by 7 kingdoms declaring war on me. I had about 2 million, so was able to buy off 2, but the remaining 5 were relentless and I stood no chance.
At no time did any other kingdom request peace, so once a war started it went on until I bought them off. There was no time once the first war started when there was peace.