What is the average signal strength of an Indoor Access Point & an Outdoor Access point? (In an open environment scenario)

Signal strength which is usually represented by RSSI, but you need to consider another factor which is the free space loss which will degrade the signal and therefore affecting the overall performance.

However, the average RSSI for both indoor & outdoor APs assuming open environment neglecting the free space loss is around - dBm.

Keeping in mind, during the design stage, and as a general rule, - dBm coverage might provide insufficient service depending on the user handset/PC.