Hi Etienne,
I don't use Maxwell but Thea which is pretty similar, and in order to use 18Gb of Ram I'd say the scene you render should have at least 60-70mil polygons and some heavy duty textures.
Ram usage is based on polys count, texture sizes, rendering resolution and multiple lights buffers when using the multilight feature.
I've recently rendered a scene with 50mil polys @ 6400x3600px and memory usage went up to roughly 12Gb if I remember well.
Hope this clarifies the matter a little.
Cheers
Francois
François Chatelain
Worldwide Digital Imaging
Formerly posting as RanXerox
"A little bump will help blur your reflections"