When u apply for telkomadsl u pay a pro-rata and a monthly. (pro-rata is for current month *say 15 days of the current month*, and u pay one month in advance)
So if u cancel, it would be put off from next month. and u won't be charged for your current month. (how quick telkom puts it off, depends tho.) Maybe they will charge u for the current month, but u will have free account for next month then or something like that if they don't act quick
I would suspect that on bandwidth related matters it would at least be till the end of the month you are in. I base this on the fact that you pay in advance for the service.
However I always confirm with the person who I am cancelling with till when the existing service will last. Although I saw my arse when it came to porting coz I never thought it would not be a seamless operation.