When did the major period of mare volcanism generally cease?

Around 3.0 to 3.2 billion years ago

While the impact basins—the molds for the maria—were established during the Moon's intense early bombardment phase within the first billion years, the subsequent lava flows occurred much later. The initial outpouring of lava began around 3.8 billion years ago, but the extensive period of mare volcanism gradually tapered off and generally ceased in the range of 3.0 to 3.2 billion years ago. This later cessation time explains why the maria appear less densely cratered than the older highlands; they have not been exposed on the surface to subsequent meteorite impacts for as long a duration as the surrounding, older terrain.