Typically a good signal is around -55 dBm. -60 would be considered "full strength" or just below full strength. Above -65 dBm and you will start to see greater UDP packet loss and lower TCP throughput. Once you get above -75 dBm you can start to have connection issues.
This holds true for me with 2.4 and 5 GHz when the noise floor is between -100 to -115 dBm