Answer based on context:

Meanwhile, in July 1718 the Spanish, this time with 30,000 men again led by the Marquis of Lede, had also invaded Sicily, which had been awarded to the Duke of Savoy. They took Palermo on 7 July and then divided their army in two. De Lede followed the coast to besiege Messina between 18 July and 30 September, while the Duke of Montemar conquered the rest of the island. The French, Austrians, and British now demanded Spanish withdrawal from Sicily and Sardinia. The attitude of Victor Amadeus II of Savoy was ambiguous, as he agreed to negotiate with Cardinal Alberoni to form an anti-Austrian alliance. On 2 August 1718, Britain, France and Austria joined with the United Provinces of the Netherlands to form the Quadruple Alliance in the Treaty of London. On 11 August, at the Battle of Cape Passaro, a British fleet, led by Sir George Byng, effectively eliminated the Spanish fleet stationed off Sicily. This was followed in the autumn by the landing of a small Austrian army, assembled in Naples by the Austrian Viceroy Count Wirich Philipp von Daun, near Messina to lift the siege by the Spanish forces. The Austrians were defeated in the First Battle of Milazzo on 15 October, and only held a small bridgehead around Milazzo. In 1718, Cardinal Alberoni began plotting to replace the Duc d'Orléans, regent to the 5-year old King Louis XV of France, with Philip V. This plot became known as the Cellamare Conspiracy. After the plot was discovered, Alberoni was expelled from France, which declared war on Spain. By 17 December 1718, the French, British, and Austrians had all officially entered the war against Spain. The Dutch would join them later, in August 1719.

Over how many months did De Lede besiege Messina in 1718?
2