Where can one find how long a cease fire has left?

Solution 1:

Go to Diplomacy Overview -> Deal History. All your current peace treaties should be listed there, along with (1) their duration and (2) their starting turn. That means you can easily find out the remaining turns via [duration] - ([current turn] - [start turn]) (a negative value means it has expired).

RE: your comment on the other answer, since the Fall 2012 patch, peace treaty duration depends only on game speed:

• Quick - 10 turns
• Normal - 10 turns
• Epic - 15 turns
• Marathon - 30 turns

Solution 2:

Cease fire peace treaty's last 10 turns. You should receive an alert on the right hand side of your screen when the cease fire period is over.

Another way you can check is via the diplomacy menu. If you click on a civilization and the 'Delcare War' option is grayed out then you are still during a treaty period.

I know there are other ways, possibly using the world icon in the top right of your screen that lists all the civilizations but that is all I can answer for now. I will test this and complete my answer.