You should get an immediate, automated response just to let you know your mail did not end up in a black hole.
Then you should get a human response within 24 hours (the SLA). In rpm's test, it came within 6 hours.
Typically you should use e-mail support for questions that's not critical, i.e. that can be resolved in the 24-hour timeframe.
For more critical questions, you should call 155. Current stats show 80% of calls are answered within 20 seconds.
Also, remember the online, selfhelp system.
Not a criticism of vodacom specifically, because a lot of companies do this, but I wish people treated email as high in priority as phone calls. If I phoned in, I'd get a response; why not if I email in?
And why, oh why, do people phone you in response to an emailed question? If I wanted to use the phone, I'd have just phoned them in the first place. Seeing as I didn't, odds are I want an answer via email. (And this one is aimed at vodacom, together with a lot of other companies).
Sorry, hijacking the thread to go on a personal rant here

But as you may have guessed, I dislike using the phone, and really dislike being forced to use it for internet-related support - after all, if anyone is going to use the net for communication, IT companies should be the ones doing it!