What JBoss version and OS you use?
AFAIK the calculation of nanos might be not exact depend on the OS and JVM :( , maybe you test it with a simple java program.
I've test the 'communication overhead' simple by using a dummy SLSB and the client call it 1.000.* times (with one and more threads) to have an idea of the average.
Also the duration depends on the payload (size of method parameter and return values).