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.