Ideally speaking, you once have the correlation of the signal with the receiver by measuring 'n' samples and then using various methodologies either root mean squares or average of peaks of 'n' samples to calculate the signal strength values.

Now, how quickly does it update? measurement frequency and measurement interval are customizable and it is technology dependent let's say you are using CDMA, or LTE etc.

Specifically for 802.11b signal strength is sought every 100 ms and at every significant event that requires updated signal strength.