Hey friend, there's a telkom adsl thread that's got all the answers to your questions.
Well let me try an assist, there's three tiers of throttling
Tier 1: when you reach 300GB, your speed will be reduced to half for a short period of time
I think it's 7pm-9pm
The numbers below are estimates since i honestly can't remember :/
Tier 2, when you reach around 450Gbs or 500Gb
You'll be throttled for a much longer time period
Tier 3, don't remember the exact number but you'll be throttled the entire day at 60kbs
You can easily avoid being throttled by setting your downloads from midnight to 7am, during night surf your data usage doesn't get added to your monthly usage.
So you can download the entire Internet from midnight to 7am.