I’m not sure anyone knows this one for sure. It depends on what volcanoes you’re thinking about. For example, in 1980 at Mt. St. Helens, the first indication that magma (we call molten rock “magma” before it reaches the surface and “lava” after) was moving into the volcano came in early April, and the eruption took place in mid-May. Certainly the magma was moving up from within the upper mantle of the Earth prior to early April, but exactly how far it had to come and how long it took to get there we don’t know.
On the other hand, there are volcanoes like Kilauea which has been pretty much been erupting constantly since 1983. There must be an almost continuously-open pathway from the source of the magma in the upper mantle to the volcano, but again, I don’t think anyone has a very good idea of how fast the magma is traveling through this pathway.