Did this ever take place? Where?

Yes. It was generally a part wherever the Latin Church was but also Byzantine.

What time period was this in effect? Why did it end?

It technically goes back a long time - the Egyptians and Hebrews had it, as did the Greeks, Romans, and Germanic tribes. Most in Europe inherited it from the Roman laws but not exclusively. I'm now going to concentrate on England -it probably had its legally secured heyday in the 12-13th centuries. Criminal sanctuaries died in England in 1624 under James I, there was a modified form of sanctuary for debtors which was abolished by William III in 1697 ('An act for the more effectual relief of creditors in cases of escapes, and for preventing abuses in prisons and pretended privileged places').

It ended mostly because it was being abused, used as a matter of course by criminals (mostly by debtors and enemies of the monarch), but also by the clergy. There is also the wish of the State to control criminal misbehaviour, rather than having the Church interfere. During Henry VII's reign, the Chief Justice remarked that sanctuaries were 'more like a den of thieves' than 'a place of protection.' It's actually not just churches - there were other designated areas, it's just that churches were automatically sanctuaries under canon law.

Under what circumstances did a church grant sanctuary?

Anyone could be safe in a sanctuary except traitors (a point that Henry VII negotiated with the papacy), recidivists (in 1467) and debtors. Henry VIII refused sanctuary for people suspected of rape, murder, highway robbery, sacrilege, robbery, or arson. During the 1530s, Henry VIII stopped abjuring the realm (see Bracton below), and made you stay in a domestic sanctuary, and during the 1540s started paring down the places you could claim sanctuary in, limiting it to churches, hospitals and colleges, mostly under the direction of Thomas Cromwell who wished to achieve 'the utter destruction of sanctuaries'.

Sanctuary is supposed to create a breathing space before hasty justice is executed, where you could make amends with your pursuers (if possible), and if not, gain the necessary protection to abjure the realm.

Was it indefinite?

Alfred the Great had it for 3 nights. The Assize of Clarendon (1166) limited it to 40 days after the arrival of the coroner, with family and friends providing food and water, but this actually isn't canon law, simply English custom. Henry de Bracton who is the main law commentator during the 12-13th centuries suggested that anyone who actually stayed in there a long time was guilty, and should be starved out quickly. The choice was to abjure the realm and

a reasonable number of days’ journeys to the port ought to be reckoned for him and he ought to be forbidden to leave the king’s highway, or to make stay in any one place to two nights together, or to turn aside anywhere, or to stay far from the highway except because of great necessity or to seek shelter. Let him always keep on the direct road to the port so as to be there on the day assigned him, and, provided he is not hindered by a storm, cross as soon as he gets a ship and a wind. if he does anything to the contrary he will be in peril.

So in other words, confess and get out the country via the port appointed by the coroner as quickly as possible, otherwise they were starved out. There were some places in England where the King could not reach, (and established by royal charters no less) and there you could stay indefinitely, such as Westminster Abbey.

To what lengths would a church go to protect the individual or abed their escape?

I'm not aware of any churches helping people escape, as this would run the risk of the wrath of the state.

Did the secular authority ever violate a church's declaration of sanctuary for somebody?

Thomas Becket is probably the most famous one, (according to Henry II's chronicler, Henry Knighton) but you could also have [Archbishop Geoffrey](http://en.wikipedia.org/wiki/Geoffrey_(archbishop_of_York)) who had sought sanctuary in the local church at Dover, and was dragged from it by Longchamp's men. The case of William Fritz Osbert (also known as Longbeard) has him smoked out of St Mary le Bow's church by the local justiciar. Edward IV, in the War of the Roses, violated sanctuary in Tewkesbury Abbey, and forced the Duke of Somerset and other leaders of the Lancastrians out of the church, and then executed them immediately in the town square.

Did any individuals of historical significance ever seek sanctuary?

Becket is probably the most famous I can think of, but the Duke of Somerset is probably a close second.

The right of sanctuary definitely existed, and was, of course, sometimes violated.

Did any individuals of historical significance ever seek sanctuary?

Lots of examples from the War of the Roses.

After the Battle of Tewkesbury, where the Yorkists under Edward IV decisively defeated the Lancasters, and many of the Lancasterian leaders, among them the Duke of Somerset took sanctuary in a nearby Abbey. But a couple of days they were dragged out and beheaded. The Abbey was subsequently reconsecrated.

Another example if Elizabeth Woodville, the Queen of Edward IV, took refuge in Westminister Abbey twice, once after the deposition of Edward IV, and another after his death in 1483.