Calculating amount of diet soda a dieter can drink without dying
Each of the three methods runs correctly, and does so in constant time.  That's good.

Comments

You wrote one comment.  Either


remove the comment (because it could become inconsistent if the constants change and is therefore a maintenance headache), or
hard-code the answer and discard everything else (System.out.println("2600.0 litres.");), or
write the comment as asssertion instead (assert maxAmountOfDietSodaDieterCanDrinkInLitre() == 2600.0;)


Constants

The names of the constants and methods are extremely clear, which is good.  You might want to shorten them, though.  If so, I suggest:

private static final double DIETER_TARGET_KG = 65,
                            MOUSE_KG = 0.025,
                            MOUSE_SWEETENER_LD50_L = 0.001,
                            SWEETENER_CONCENTRATION_IN_SODA = 0.001;


That puts the members in an order where each constant is related to its neighbour.  As a bonus, they are in alphabetical order.  I've chosen to use SI units throughout.  "LD50" is standard notation in toxicology for the median lethal dose.

Methods

I think you could have decomposed the problem more logically.  For example, what quantity is being expressed by the parenthesized expression

(maxAmountOfDietSodaAMouseCanDrinkInLitre() * DESIRED_WEIGHT_OF_DIETER_IN_GRAMS)


… whose units are L ∙ g?

I would therefore write the solution as follows:

/**
 * Median lethal dose of artificial sweetener, in litres per kilogram of
 * the subject's body weight.
 */
public static double sweetenerLD50() {
    return MOUSE_SWEETENER_LD50_L / MOUSE_KG;
}

/**
 * Median lethal dose of diet soda, in litres per kilogram of the subject's
 * body weight.
 */
public static double sodaLD50() {
    return sweetenerLD50() / SWEETENER_CONCENTRATION_IN_SODA;
}

/**
 * Median lethal dose of diet soda, in litres.
 *
 * @param subjectKG subject's body weight, in kilograms
 */
public static double sodaLD50(double subjectKG) {
    return subjectKG * sodaLD50();
}

public static void main(String[] args) {
    System.out.println(sodaLD50(DIETER_TARGET_KG) + " litres.");
}


Note that unlike the // 2600.0 litres comment, JavaDoc serves as useful documentation, and is unlikely to become out of sync if the numbers change.