Realistically only about half an hour. There are instruments at the L1 lagrange point that can measure the properties of the solar wind and/or CME as they pass

Realistically only about half an hour. There are instruments at the L1 lagrange point that can measure the properties of the solar wind and/or CME as they pass and allowed real informed responses to events. There are also very, very simplified CME propagation models (WSA-ENLIL, etc) that can start with some assumptions and give a reasonable idea about when the CME will arrive within ~12 hours. But for every time the simulations succesfully predict severe geomagnetic events or the like they give false positives a couple dozen times. The simulations cannot predict the geoeffectiveness.