I have also had very fast fermentations. In fact, I just bottled a porter yesterday that was brewed one week ago. fermentation started in 4 hours, I hit my target SG after 2 days, I let it sit for another 3 days, checked my SG again. Bottled after a week.
But, as the other brewers have said, a hydrometer is needed to check gravity to insure fermentation is complete.