The length of the ram preview depends on the available ram. The previews are uncompressed so even if your original footage is a highly compressed mpeg stream with a data rate of 3 Mb/s your ram preview will read every pixel.
There are tricks to extending ram preview length. You can skip frames, reduce the size of the comp window with resolution set to Auto, or set the resolution (both options are in the Preview panel) to a lower rate.
It looks like you have 6GB of ram. 16 seconds sounds about right.
I'm pretty new to AAE so can you tell me the steps to skip frames and the other things you mentioned? Also is there a way to listen to the sound plus clips without doing ram preview I really wanna be able to see all of it. And my final question is, is there a way to chose a certain point in the video where I want it to show me the ram preview, like say the last 16 seconds of the video instead of the first 16 seconds. I know that's a lot of questions but I really wanna know how to get the full potential out of the program, thanks!
You've gotten a great answer.
This page from the FAQ has some more info too: FAQ: Why does RAM preview only play part of my composition?
This sort of thing is fairly basic AE stuff. Since you're new to AE, I would highly suggest you start here and go through all of the available (free!) resources there. AE is a rather broad AND deep program, you can't really do well in it unless you have a solid understanding of the basics.
Again, to avoid a lot of future frustration, go here.
Thank you both very much