It's not normal. The duration should be about equal, though, to be honest, 64bit apps are "fatter" to begin with and consume more RAM. The RAM preview being so short would indicate one of the following problems:
- decode issues with compressed footage due to missing or incorrectly installed CoDecs
- also related to compressed formats: shortage of disk space for the media cache
- audio hardware issues
- graphics card misconfiguration
- general issues with drivers not running properly in 64bit
Check all these things and especially check your hardware configuration (see here for some guidance). i realyl suspect it's a configuration issue outside of AE that forces your system in quirks mode and impairs 64bit-ness.
AE CS5 is even more efficient thant CS4 with low ram amount. You should have slightly longer Ram previews in CS5 than CS4 with 4gb of ram.
now, i know the answer:
You need an internet-connection and allow the "dynamic link processor" to connect the internet. Then you get the full ram preview....
It sounds a little bit funny, but it´s so....(I tested it several times)
That Dynamic Link process is not connecting to the Internet. Rather, it uses TCP (the protocol over which internet communication runs) to communicate internally with the main After Effects process.
That said, you are experiencing something unexpected if the Dynamic Link process is failing.
Please file a bug report to tell us what exactly you did to affect your RAM preview, giving as much detail as possible.
Also, try this: Start the computer in Safe mode. Then run After Effects, checking to see how long your RAM preview is. If it works better, then try restarting the computer in normal mode again and running the same test. Oddly, we've seen some cases where this clear up some strange behavior.
As I said, filing a detailed bug report will help.