Medieval post is really not my area of expertise, but I can give some indication of roughly how long messages would take to travel between the Holy Land and Europe based on some evidence we have. Messages would primarily have traveled by ship, the overland route took months and was very dangerous. They type of message could have a significant impact on how long it took to travel back as well. A personal message, for example, might only be entrusted to a close friend and would have to wait until that individual traveled home while a general piece of news or a letter to the papacy could travel back via an Italian merchant or general member of the clergy.

The short version is that important news could travel from the Holy Land to Italy in a couple of weeks. One of the best examples to look at for this is the single biggest piece of news from this period: the fall of Jerusalem. Jerusalem fell on the 2nd of October 1187. News of the city's fall has, by some chroniclers, been listed as causing the death of Pope Urban III. The Chronicle of Ernoul actually says that it was news of the defeat at Hattin (4 July 1187) that shocked Urban to death, but this seems to be a bit of an exaggeration on his part. Whether or not the news of the fall of Jerusalem actually shocked Urban III to death is highly debatable, but we do know he died on the 20 of October 1187. That gives us a period of 18 days for news of the loss of Jerusalem to reach Rome.

Now, this should be seen as about as fast as possible for news to travel between these places. Rome is relatively near to the Holy Land, at least in terms of not being north of the Alps, and this news would have been of the highest priority. As a point of comparison, when the First Crusade took Jerusalem on 15 of the July 1099 news of their victory did not reach Europe before Urban II's death on 29 July 1099. This comparison isn't perfect of course, by the Third Crusade travel between the Holy Land and Europe as much more common so messages would have traveled faster, but it is still enlightening.

Thomas Asbridge's The Crusades is probably my favorite general history of this subject.