I play until I have met all the civs in the game. Usually by then I dominate my continent. If I am ahead on points over all by then I figure I win and play a new game . Otherwise I keep playing until I knock down whichever civ was leading
Out of curiosity, if you find it that easy, why don't you bump up a difficulty level. What you are describing is what Prince feels like to me. By the time I've met all the Civs, I'm well in the lead, and haven't lost a Prince game in a long time. But I jumped up to King, sometimes I even play Emporer, and though it took me a bit of adjusting, I now do quite well at King and reasonably well at Emporer. More fun for me.
Anyways, to answer your question I had to ask the above question. If I were to keep playing at Prince level, and I get a points lead around 1000 AD, I win all my games. But it is POSSIBLE to lose to the AI if you don't pick a victory condition and follow through, or attack the other civs so that they don't have the wherewithal to mount a serious points challenge and can't get a victory condition on their own.
But on King, I can get a pints lead, hold it for quite a while and still lose because the AI can focus very tightly on one victory condition, preventing it from scoring points in a broader sense on the scoreboard, and then win with an achieved victory condition without me having pursued one as quickly.
TL; DR - Unless you do a lot of conquering/razing, the AI can beat a human who is ahead on points by focusing singularly on a victory condition. But a human with a huge lead should win no problem as long as they focus on a VC.
