When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation (west or east), for Mercury, is between 18° and 28°; and for Venus between 45° and 47°. This value varies because the orbits of the planets are elliptical, rather than perfect circles. Another minor contributor to this inconsistency is orbital inclination: each planet's orbit is in a slightly different plane.
Refer to astronomical tables and websites such as heavens-above to see when the planets reach their next maximum elongations.
In 2008, Venus does not have a greatest elongation - either eastern or western. The planet instead moves from a greatest western elongation on October 26, 2007 to a greatest eastern elongation on January 17, 2009.
In 2008, Mercury has greatest eastern elongations on January 21, May 14, and September 11 (and after that on January 4, 2009). Western ones happen on March 3, July 1, and October 22.
For example, Venus's year (sidereal period) is 225 days, and Earth's is 365 days. Thus Venus' synodic period, which gives the time between two subsequent eastern (or western) greatest elongations, is 584 days.
These values are approximate, because (as mentioned above) the planets do not have perfectly circular, coplanar orbits. When a planet is closer to the Sun it moves faster than when it is further away, so exact determination of the date and time of greatest elongation requires a much more complicated analysis of orbital mechanics.
All superior planets are most easily visible at their oppositions because they are near their closest approach to Earth and are also above the horizon all night. The variation in magnitude caused by changes in elongation are greater the closer the planet's orbit is to the Earth's. Mars' magnitude in particular changes with elongation: it can be as low as +1.8 when in conjunction near aphelion but at a rare favourable opposition it is as high as -2.9, or seventy-five times brighter than its minimum brightness. As one moves further out, the difference in magnitude caused by the difference in elongation gradually fall. The maximum and minimum brightness of Jupiter differ by only a factor of 3.3 times, whilst those of Uranus - which is the most distant Solar System body visible to the naked eye - differ by a factor of 1.7 times.
Since asteroids travel in an orbit not much larger than the Earth's, their magnitude can vary greatly depending on elongation. Although more than a dozen objects in the asteroid belt can be seen with 10x50 binoculars at an average opposition, only Ceres and Vesta are always above the binocular limit of +9.5 at small elongations.