It is impossible to answer this question unambiguously because in each timepoint the distance from Earth to Mars will differ. Nevertheless it is possible to give extremely exact answer. And moreover, to consider its great practical value for future mankind
Theoretical consideration of a question
It is impossible to answer this question unambiguously because in each timepoint the distance from Earth to Mars will differ. It is explained by the fact that planets of the solar system are in the constant movement around the Sun (if they did not rotate around a star, then simply would fall to its heated surface, attractions of our star taken by huge force), besides the speed of their rotation is different.
Planets will be at the minimum distance from each other (it approximately the 55th million kilometers) when Earth is on one line between the Sun and Mars. Such position of planets is called "opposition", and there is it approximately every two years. The distance between Mars and Earth will be the greatest when the Sun is between these two planets on one with them lines. In this case the distance between planets will equal to about 400 million kilometers.
Practical value of a question
Though Mars is only the planet, the second in proximity to Earth (the superiority belongs to "a morning star" here - Venus), nevertheless it became the most probable candidate for priority development and colonization by mankind. Unlike Venus which temperature on a surface reaches +500 degrees, intolerable for people, and pressure 92 times bigger than terrestrial – Mars has very tolerant conditions. On the equator of "the red planet" temperature rises up to +20 degrees, pressure is less terrestrial and also at the planet there is water. Besides, unlike the same Moon, an attraction of Mars rather strong to hold the atmosphere. Thus, first of all these factors explain the considerable interest of people of Earth in their red neighbor shown from the middle of the last century in sending from Earth of various research stations and robots mars rovers. The foundation for this process was laid in far 1960th year by the Soviet Union which sent the first to Mars the spaceships and the first gone down on its surface. Certainly, economically to send to Mars messengers from Earth, only when the distance between planets is the smallest – in this case technologies at the present stage of development of our civilization allow spacecrafts to reach Mars approximately in 150-300 days (at their average speed 20,000 km/h); the exact amount of time in way depends on the speed of start, a route, position of planets, fuel quantity and the useful equipment onboard. But such term is still rather big to send human crew to Mars even if on the shortest way. Duration of space flight for people becomes more than 250 days dangerous in view of constant action on them the background radioactive radiation which is present at interplanetary space. The big danger is represented also by flashes and storms in the Sun which can ruin future astronauts in few hours. Therefore the issue of reduction of time of overcoming interplanetary distance between Mars and Earth is still very topical.