We don't really know for certain. Estimates from the geologic record suggest that a reversal can take less than 100 years in some instances. The effects upon the biosphere seem to be minimal since no paleontological evidence for high mutation/speciation rates correlate with the known reversals. As for humans, we have no remains from 250,000 years ago or so to look at in any detail. The major impact would be from solar activity and the apparent loss of the Earth's protective magnetosphere during this time. However, the Earth's atmosphere is so thick that it may be unlikely that earth life is even affected in the slightest!!
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.