A face mask is required:
On June 9th the Royal Decree-Law 21/2020 PDF, on urgent prevention, containment and coordination measures to face the health crisis caused by COVID-19, has made the use of masks for people over 6 years of age mandatory. In any situation where it is not possible to maintain an interpersonal safety distance of at least 1.5 meters and in any means of transportation a mask covering the nose and mouth is required. Regulations may vary depending on the local regulations. In some places, a mask needs to even be worn outside on the Camino.
A mask should be worn a max of 7 hours and then replaced.
You can find out about which mask you should use in this Guide of the Ministry of Consumer Affairs