After effects is using way too much memory rendering...

For one thing you should ignore the estimated time remaining because it calcualetes based on whats rendering that instant.  So if rendering a particualry expensive frame it will estimate the entire remaining project based on that frame,  So it might estimate 3 hours and then three minutes later estimate 3 minutes, or vica versa.  Other than that without knowing specifics about your system and your project and what program you are rendering with and what your render setting are, nobody can really give you any specific advice.   Like Dave said.  Be sure to limit the amount memory that AE is allowed to use, because it will use it all if you let it, leaving none for other applications, including your operating system.

What is going on in your comp? I have comps that take one or two minutes a frame to render. I have other comps that render at a half dozen frames a second. It all depends on what is going on in the comp. The RAM usage varies as projects render. Are you talking drive space from the rendered file or are you talking ram from your system monitor? 18 seconds of 4K uncompressed could easily be over 30 GB. I'm running a ram preview on a 4K comp right now and working on the forum while it renders about 4 seconds of my comp so I can check a transition. This is my Ram Usage:

The usage goes up then comes down again. When the work area is rendered usage drops a bit. The same thing happens when I render using a BG rendering app, or the Rener Queue, or the Media Encoder. Memory goes up and down as the workload changes. 

After Effects eats memory. All of it, given half a chance. If that's a problem, you can restrict the amount available using the preferences > memory panel.