It is generally good to keep the monitors at more than 1.5 m from each other. By pointing their tweeters (the smaller, mid-high frequency loudspeakers) towards your ears you should form an equilateral triangle with the lines connecting the center of your head and the tweeter centers and the line between the tweeter centers themselves.
In any case the angles at the monitors must be equal and should never exceed 70 degrees.
The monitors should have the same height from the floor: between 1,20 m and 1,90 m. They should never lay on the mixer meter bridge because the direct sound from them would interfere with the mixer surface too much, it is better to put them on independent stands behind the mixer so that they are well in sight by the listener.

It is important to keep a left-right symmetry in the room: the listener will have the same space at his left side and his right side. The same will happen for the monitors, the distance between the right monitor and the right hand wall will be equal to the one between the left monitor and the left sidewall. The ideal lines connecting the ears to the loudspeakers should never have any object interrupting it.

These advices will equilibrate the listening experience on both ears and will permit to use our psychoacoustics abilities on locating sounds at best. Do refer to the monitors’ manual to compare any difference your own equipment might require from the generic advice that what was just given.

Another important thing to remember is to move away the monitors from the back wall: this prevents a comb filtering effect at low frequencies (the worse effect is a few quite definite notch filters in the bass region). 90 cm are sufficient if you work on human voices. For the same reason monitors should be about 70 cm from the side walls, but we will answer more deeply on this question in another occasion. Anyway a good general advice here is to put sound-absorbing materials on the area on the walls right behind the monitors.
Note that all of these concepts are good for positioning also the frontal couple in a 5.1 surround system.

Remember that all of this is just a part of the acoustic correction project of a room: when a scientific approach is taken, these positions will be studied in relation to the acoustic treatment on the walls, the room dimensions and the ergonomics of the space itself.