250 grams is how many milliliters?

This is impossible to answer without knowing the density of the substance you're talking about - a gram is a unit of mass, whereas a millilitre (or milliliter) is a unit of volume. For water, 1 gram occupies 1 millilitre - so 150 grams of water is 150 millilitres.

If the density is measured in grams per millilitre (approximately equivalent to kilograms per cubic decimetre), then volume = mass / density.