Final answer:
The provided reference materials do not give the specific date when Richard's army began marching down the coastline to Jaffa during the Third Crusade, nor do they mention the length of the march. Additional historical records are needed to provide an accurate answer.
Step-by-step explanation:
The student is asking about a historical military march conducted by King Richard I of England, also known as Richard the Lionheart, during the Third Crusade. However, the provided reference materials do not specify the exact date Richard's army began marching down to Jaffa nor the duration of the march. In the absence of the specific information on the question, we can infer from the historical context that Richard I was active in the region after the capture of Acre by the crusaders in 1191 and before the treaty with Saladin in 1192. The coastal march to Jaffa would have been part of the crusader efforts to secure hold of the coastline of the Holy Land, but precise details like the exact date and the length of the march are not provided in the given references. To find a precise answer, additional historical sources and records would need to be consulted.