This map shows the retreat of Cornish language speakers over time. This wiki page also has a chart - which is largely fanciful - describing the number of Cornish speakers and population size. This is more estimate than fact.

The Cornish language was declared dead in the late 1700s, but it was declared dead several times with the deaths of several individuals, and almost always, people would say that with the death of that person, very few people could understand what she (or he) said - indicating that language comprehension lingered. Quickly on the heals of the "death of the language" (whatever that means) there was a revival movement. There are now about 500 people who claim Cornish and there are daycare programs and other approaches to raising children in the Cornish language. In addition, Cornish vocabulary and syntax lingers in local dialect.

All of that said, people in Cornwall were "mainly speaking English" by the mid 1700s - earlier the farther east one went.

edit: Meur ras (i.e. thank you - in Cornish) for the question.