The speed you refer to is really the throughput at a point in time. It depends on every single hop in the network from the phone to the server you are communicating with. Wait 2 seconds and the throughput may be very different.
The only way to give an indication is to download a piece of data from your host and measure the throughput. This is not super straight forward either because you need a piece of data that is large enough to represent a meaningful test but also small enough to save bandwidth.
This is stuff you do only if you really really have to.