Depending on the codec used, from 5.4 kb to 64 kb.if you use 64 kb ,you are running without compression, the same way that carriers had been doing it for 30+ years. if you use lower bit ratio ,you are compresing the voice, with the unavoidable decrese on quality.
when you see that you are running 8 kb codec, remember that there is some overhead.
On real life the normal bandwith goes from 10-12 kb per call and up to 15-20 kb with the overhead.
At those rates you will be hard pressed to notice any diference on the quality of a call versus the normal PSTN network.