When will the promotion come to an end?

No clear indication.

Telkom gave no clear indication when the promotion will end but only indicated that it will end when they can determine your behaviour patterns in terms of local ADSL bandwidth usage. You can be sure of one thing: It won’t last forever.

In other words, like previously advised here at ADSL South Africa (Broadband South Africa), do not make the mistake of locking yourself in a long-term contract for ANY short-term gain, especially when Telkom is involved.