I haven't done it yet, but I'll be taking 3.5 months for the Colombia/Ushuaia portion when I do. I'd rather have 6 months but you have to work with what you have.
Couple years ago I rode 10,000km in 3 weeks. Pavement only, in North America. It was fun but the pace was silly and I smelled very few roses (plenty of pig farms). I can't imagine 4-5 weeks in S.A. covering 10,000k would be very enjoyable.