Hello Awais,
As you have seen in the DN505 on page 5, the RSSI Response time depends not only on the AGC state, but also on the signal strength. For most design notes and test procedures, they are carried out under optimal conditions, that is very little interference, the highest signal strength possible and the MINIMUN distance.
Getting the exact same results as in DN505 would require you to reproduce there exact test procedure. That would mean using a CC2500 device, not a CC430. Figure 2 and 3 on page 6 show the varying RSSI values under different conditions.
I am not familiar enough with how the RSSI_VALID signal is generated to give you an accurate response. Unless the conditions on page 5 and the RSSI threshold cause the RSSI_VALID to never get set, I don't see why you couldn't use it as an interrupt flag.
Thanks,