The burst lasts as long as it takes the stuff (radiation) get past the planet. That is only a few seconds. That radiation is travelling at light speed so given

The burst lasts as long as it takes the stuff (radiation) get past the planet. That is only a few seconds. That radiation is travelling at light speed so given our present knowledge an early warning system is unthinkable. However for a star big enough to go supernova, expected life expectancy and spectral analysis can give you some estimate rounded to the 10s or 100s of millions of years.