Let us quote from a Digi datasheet, the description of the command used to get RSSI.
DB (Received Signal Strength) Command
<Diagnostics> DB parameter is used to read the
received signal strength (in dBm) of the last RF
packet received. Reported values are accurate
between -40 dBm and the RF module's receiver
AT Command: ATDB
Parameter Range [read-only]:
0x17-0x5C (XBee), 0x24-0x64 (XBee-PRO)
Absolute values are reported. For example: 0x58 = -88 dBm (decimal). If no packets have been
received (since last reset, power cycle or sleep event), “0” will be reported.
Check the specific datasheet for your module to get exact values.
We can tell you that usually goes from -40dBm (good) to -90dBm (poor).