Are you sure you're in the correct forum? Premiere Pro doesn't have RAM Preview, After Effects does.
How big are the images? What size is the comp? Did you import as a sequence or trim the layers and use the keyframe assistant to sequence them? Have you done this kind of project before?
My standard way of working with time lapse footage from stills is to use Lightroom to resize the original images to the size they will be needed in the comp so that they are at 100% scale at tome time in the timeline. Usually for an HD comp this means I resize the images to about 2500 to 3000 pixels wide so I have some ability to do some moves on the footage. Then I import as an image sequence. The frame rate I want to use for my final render is used as the frame rate for the image sequence. It's usually 29.97 because this reduces the chance of judder in camera moves or motion in the frame and it is my preferred frame rate. I then drop the image sequence in a new comp that is the size and frame rate for my final render, make sure the composition preview resolution is set to Auto and start the comp.
If you have very large images imported as a sequence and scaled down to something like 20% to fit in the comp and you have the magnification ratio of the Comp Panel set to 50% and the Comp Resolution to full you'll probably only get about 2 seconds of ram preview.
I hope this helps.