A lot depends on the viscosity of the lava, the terrain on which it is emplaced, and the rate at which it is being erupted to the surface.
Channel feeding aa lava flow, 1984 eruption of Mauna Loa. Photograph courtesy of U.S. Geological Survey, March 25, 1984.
The longest recent flow in Hawai’i was erupted from Mauna Loa in 1859. It is about 51 kilometers long from the vent to the ocean (we don’t know how much longer it went out under water, but probably not too much farther). There is a lava flow at Undara in Queensland, Australia is 100 miles (160 km) long.
Cross-sectional view of lava flows of the Columbia River flood basalts, part o! the lava plains of the western United States. Note wake from small boat near left margin. Photo by Steve Mattox, July 1989.
There are lavas called flood basalts that are hundreds of kilometers long. We don’t know too much about how fast they were flowing. Some folks think they were fast-moving, other folks think they were slow. This is an area of volcanology where lots of research is currently focused.
This image shows Mare Humorum and the western edge of Mare Nubium. Mare Humorum is a small circular mare on the lunar nearside. It is about 275 miles (~440 km) across. (Earth-based telescopic photo from the Consolidated Lunar Atlas)