So, the thing is we can’t really predict CMEs. What we can do is observing them by satellites like SOHO or SDO (actually you can also check these data, check for jHelioViewer). While light indeed needs roughly 8 minutes to travel from the Sun, the ejected plasma is much slower than that, it travels at a comfy pace of 1000km/s or so. So it takes about a day to reach Earth (you can do the precise numbers as a homework or use Wolfram alpha to cheat), so from the observations we have roughly this time to do anything.
So, the thing is we can’t really predict CMEs. What we can do is observing them by satellites like SOHO or SDO (actually you can also check these data, check for jHelioViewer). While light indeed needs roughly 8 minutes to travel from the Sun, the ejected plasma is much slower than that, it travels at a comfy pace of 1000km/s or so. So it takes about a day to reach Earth (you can do the precise numbers as a homework or use Wolfram alpha to cheat), so from the observations we have roughly this time to do anything.