There's never any limit on how long Hamas can fire at Israeli civilians: at worst that produces only "concerns." But let Israel launch an operation that looks like it might be successful and the clock starts running:

The army believes the incursion into Gaza will do significant damage to Hamas' standing army and at the same time give Hamas leaders a palpable sense that their rule is in danger. The ground invasion will also accelerate the diplomatic stopwatch. A delegation from the European Union "troika" (Germany, France, Great Britain) will reach Gaza on Sunday, and French President Nicolas Sarkozy is expected on Monday. Translated into military terminology that means the IDF has less than a week to make genuine progress in Gaza.

A proposal to get the UN to declare a ceasefire was quashed by the United States. VDH's description of the situation seems apt:

There is something especially nauseating about the latest Middle East war -- scenes of worldwide Islamic protests with photos of Jews as apes, protesters (in Ft. Lauderdale, Florida of all places!) screaming about nuking Israel and putting Jews in ovens, parades of children dressed up with suicide vests and fake rockets, near constant anti-Semitic vicious sloganeering, Gaza mosques stuffed with rockets to be used against civilians -- all to be collated with creepy Hamas rhetoric about the annihilation of Israel. This is the world in which we now live.... It is now clear that the so-called and much praised "international community," the hallowed U.N., the revered EU, all pretty much are indifferent to the survival of a democratic Israel, or are actively supportive of its terrorist Hamas enemy. Only the U.S. (for now) stands by a constitutional state in its war against a murderous terrorist clique, with annihilation its aim and religious fascism its creed.

Ditto from Roger Simon and Mark Steyn.

References:
*ANALYSIS / Israel's aim in Gaza is to break Hamas resistance [Ha'aretz]
* U.S. quashes Arab-backed Gaza cease-fire resolution in UN Security Council meet [Ha'aretz]
* Creepy Times [VDH / NRO]
* Israel Alone [Roger Simon]
* Mark Steyn: Gaza has its version of rocket scientists [Steyn]

Previously:
* Concise List Of Middle East Failures That Were Going To Happen Anyway But That Foreign Policy Experts Will Now Blame On Israel
* Israel Launches High-Tech Public Diplomacy Outreach To Explain Operation Cast Lead
* EU Diplomats: If Israel Stops Defending Itself We'll Fulfill Some Of The Security Commitments We Screwed Them On Last Time. For Real! Promise!