Not all seems to have read our blog
We have reached Capetown inside of two months (52 days) - and we do an extra line to botswana, with additional 4500km - and have done severall resting / chilling days, and also many days in Nationalparks.
We would do it again with a two month timeframe... But the costs of the whole trip are the fuel, visas, insurance and road tax, when you do wildcamp most of the time (what you have to often (no Hotels).
So it is near the same price to do it in 2 or 4 months, the food is very cheap.
I suggests to start to watch for the things you want to see in each country, make a list of attractions and cities and so on - you want to see.
Than do you can start planning, how much time you will need. I think 3 months let you see everything you want, and give you also time to relax.
With 4 months you have a very comfortable timeframe, which also allows to do some voluntary work.
When you want to stay in a small village for some weeks, look how they life and try to give them a better life (dig for water and so on) - yes - you will need more time.
We will start again, when we have a 2 month timeframe.