thx for the comments. You are right about the render being a bit dark. I'll make it brighter, bump up the ambient value of the wall's shader or add some more lights.

As to why the rendering is taking so long...it has to do with Ray tracing, having so many transparent surfaces (outer glass, inner glass, plastic tube) reflecting/refracting off each other, and having such high (actually low) sampling quality values to get smooth reflections/refractions without artifacts. Anyways, i'll add an update once the fluid is added.