How do you decide when to leave a party?

I just came back from a party. Well, not really a "party" but just some drinks with colleagues at the boss' flat. These kinds of "drinking scenes" always go on forever. Someone tries to leave, everyone will convince that person to stay a little longer. It's a never-ending circle. You get up to leave, someone says "it's still early (at 4am), just sit" and then you sit, get yourself another drink. You get done with this drink, you're determined to leave this time. You've even called a car to pick you up. You go to the host (the boss) to tell him that you're leaving. AGAIN! "Stop acting like a fool, just sit". This is when I start thinking - how does this always happen, how do I always fall into the trap. If you're wondering, this luckily didn't happen today. We (another colleague of mine) escaped. This is starting to sound like I'm trying to escape from something horrid and 'not funn'. These "parties" are great funn, it's just that sometimes you just want to leave, get home, sleep or watch an episode of Weeds or Parks and Recreation. SO today, while on my way back home, I thought - how do you decide when to leave. Wait for someone to leave before you? And another one? This could take ages. (I just realised, I somehow leave the party last. Hehe.) You can't really decide can you? It was so much easier when you had curfews. Haha. Not better, but easier. Any suggestions on how you decide? I'm still thinking, trying to come up with something.