Bright enough to see the road under any conditions, and bright enough in back to be seen from a few hundred meters.
Since many of us use generator lights, they're not usually measured in lumens (raw intensity), but rather in lux which is a rating of intensity over a given area instead. A poorly focused high lumen lamp can provide a lower lux intensity than a well focused lower lumen lamp.

For me, I've found a good setup to be my Supernova E3 headlight/taillight wired combo, plus 2 PB Superflash lights in back for good visibility when riding on highway shoulders, and a Princeton Tec Quad attached to my helmet for map reading, spotlighting, and nighttime repairs.