"extensive conversations" is a correct and usable phrase in written English. For example: "Before making a decision, we had extensive conversations with all the stakeholders.".
"We never had extensive conversations about that.
"I haven't had any extensive conversations with him".
"There are no extensive conversations," Mr. Ward said.
He carried on extensive conversations with other Twitterers.
The Jets' coach also said that he had an extensive conversation with the receiver.
Asked if Trammell seemed unhappy, Torre said he sensed nothing unusual in their last extensive conversation 10 days ago.
And yet its most gripping moments involve an extensive conversation between a communist militia and the people of a Spanish town about the merits of collectivism.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union