We stayed for five nights but would have been happy to stay for longer.
Cape Town has so many interesting things to do - Robben Island and Table Mountain obviously, but Kirstenbosch Gardens are magnificent, a 'free' walking tour very informative and a trip to Cape Point is worth the (minimal) effort. Having a hire car helped but when we needed taxis the concierge service conjured up well-priced ones quickly - so useful.
Finally, when we wanted to eat in the close by supermarket was cheap and perfectly adequate (if, perhaps, a little limited in scope).