You have two starting points. If you want to see everything, you can start at the foot of the mountain. You first walk for an hour through a bushy area. Here you can spot beautiful birds and warm up your muscles for the ascent.
Would you rather start climbing right away? Then you can do the first part by car. You only have one route for the car and you end up at a parking lot. From there you can also start the climb. On average it takes you an hour from this point to get to the top of the mountain! Of course this depends on your condition etc.
Make sure you have enough water and put on sturdy shoes. The trip to the top is fun to do and in some places you have phenomenal views. Once you reach the top you can enjoy an impressive view!
The Christoffelberg is really worth it. It takes a while but the view is beautiful. Tip: Leave on time so that you can go up the mountain at about 06:30, then it is still doable with the heat.