So I made some stuff in ae CC 2017 and it's quite heavy to process, it's alot of animated 3D things from Element 3D.
When I wanna see what i' ve done so far I just press the '0' key on my numpad so its starts RAM rendering or whatever.
But a few seconds in it stops and starts playing on repeat what it's rendered so far, whilst my comp is like 30 seconds that I wanna watch, it keeps stopping at like 3 seconds and then playing those 3 seconds smoothly over and over.
How can I set it so it renders the whole comp instead?
You are running out of RAM. A preview basically renders your frames and caches them to RAM. Once you are out of RAM, it will just start playing back what it has.
So, you have multiple options. One is to add more RAM.
And/or, you can lower the resolution of your composition to, like, quarter, or something. That will use a lot less RAM to preview each frame, so your preview will go a lot longer. It is a waste of your time to get a full-quality, full-frame preview just to check the motion on things. Most AE pros do not preview at full-res except for a few frames here and there until time to render.
Speaking of...another option is just to render out your scene via the render queue or sending it to AME.