Venus spends about as much time in the morning sky (west of the sun) as it does in the evening sky (east of the sun), about 290 days. However, the amount of time when Venus is plainly visible in a dark sky can vary greatly.

It depends in part on the season. In the spring, the ecliptic rises more steeply from the sunset horizon than in the fall, so Venus will appear higher and be visible in the evening sky later than it would in the fall at the same angular distance from the sun. The reverse is true for Venus' appearances in the morning sky.

Another factor is the fact that Earth and Venus orbit the sun in slightly different planes. Thus, Venus will sometimes appear to the north of the sun (have a more northerly declination) and sometimes to the south. If Venus is north of the sun, the Northern Hemisphere will have a better view (Venus will be higher above the horizon) than the Southern Hemisphere, and vice versa.
Source(s):
http://www.michielb.nl/maya/venus.html
http://www.venus-transit.de/PlanetPhases/gifs/myVenusDiagram.gif
http://www.datasync.com/~rsf1/vel/1918ics.htm

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