6.5 ns is probably long enough. Did you actually calculate the average over 6.5ns? CHARMM prints the number of steps over which the average was calculated just before the LAVE> lines.
You may have to calculate the real long-term average yourself from the data in the logfile; there are examples in the Script Archive that show how to do this.

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden