You do not have enough RAM to show more than that. Uncompressed HD video (which Is what After Effects creates in its compositions) is very large. More info here: FAQ: Why does RAM preview only play part of my composition?
It sounds like you have the composition panel set to full resolution and the zoom factor set to something else. Try setting the Comp Window panel to auto and the zoom factor to 50% or even 25%. Also, make sure that your machine is properly optimized for AE, that your disk cache and memory cache has been recently purged, and, most importantly, make sure that AE is up to date. There were ram preview issues on some machines with some configurations in previous builds that are now solved for almost all users.
Turn off MP rendering for RAM previews. Locking the existing cached frames and/or decompressing your compressed footage will take longer than letting it calculate on just 1 processor/ core.
Thanks for the answers.
6 GB of RAM allocated for After Effects. It is more than enough to preview much longer fragment of the composition, even if the source video would be 4K, isn't it? 23 GB of disk cache is set up. Emptying Disk Cache I noticed there were only about 86 MB [not GB!] of data – it's 0.37% of allocated 23 GB! Inspite of all these settings looking like being enough to handle the process properly, the RAM preview fragment remains very short. At the moment I use a trial version (184.108.40.206) of CC After Effects.
Again, thanks for the help and information.
6 GB of RAM allocated for After Effects. It is more than enough to preview much longer fragment of the composition, even if the source video would be 4K, isn't it?