Maximum DSL speed is a function of distance, the gauge (thickness) of the phone wire used in your locale, and the DSL technology employed. The fastest DSL now commonly available is ADSL, and it offers maximum download speeds of up to 7.1mbps and around 1.1m upload. Most residential ADSL is limited to 128 - 1024 kbps for download, depending on the provider. SDSL currently has a max of 1.5mbps for both up- and downstreams being symmetric.

Rule of thumb for converting speeds to something easier to understand: take the kbps, and divide by 10. This is the maximum transfer K per second you are ever likely to see. More accurately, divide by 8 and take off 13%, which is overhead from TCP/IP and, more importantly, ATM overhead. (DSL lines use ATM as an underlying data-transport protocol).

JerboaKolinowski informs me that

i believe in lab conditions speeds of up to 50Mbps are acheivable (though not with standard kit, I assume) The max speed offered by a provider will depend on commercial factors, and also their guess at what can be consistently achieved over their customer-base (to some percentage of that).

Sources : , memory and jk.