Robmac sent me a pm with this question. I post it here, so that other users can contribute.
My query follows thus: how long do you reckon it would take to get from Buenos Aries up to Lima, and what are the best things to do and see along the way. I'm reckoning on about 6 weeks, but I've never been and I was hoping you would be able to give me a better idea?
I'd say that 6 weeks for Lima-Buenos Aires is rather tight, but it can certainly be done if you don't mind travelling longer distances at a time. The route that offers most sights would lead via Cusco/MP (take 10 days at least for that, including acclimatising to the altitude), from there to Puna/Titicaca, La Paz, Oruro, Uyuni, Salta; bigger and smaller detours east and west can be made, particularly in Bolivia (Yungas, Sucre, Potosi), and to a lesser extent in Argentina (Cordoba; Tucuman, Cafayate. I personally really enjoy the north of Argentina, but it being your first time, you have bigger sights to see ).
Well, I guess 6 weeks is not to tight at all, but it all depends on what you want to see and how long you want to visit places.
Of course, there is much to see and do on a trip like that, but if you just focus on some parts you can enjoy a 3 or 4 week trip as much as a longer trip. Things can get boring after 5 weeks actually, certainly when you keep on travelling and seeing things every single day.
Altough with 3 or 4 weeks you might need the occasional flight (sorry Niels! like I did. Didn't feel rushed at all, because I knew I just wanted to focus on some parts: salta and surroundings, southwest bolivia, Cuzco and MP. And just a brief visit to lake titicaca, which is rather dull. Also La Paz is just soso, but I am not a real big city lover.
Michael's right of course; I phrased it in my usual crappy way, but if you're satisfied just visiting the major sights on the route, 6 weeks is enough.
Personally I don't see the point in taking less time; even with the flights, 3-4 weeks turns your trip into an awfully time-pressured endeavour, with the added disadvantages of environmental pollution and you not being able to take in the scenery, which is lovely along all of the itinerary. Sticking to ground transport, as usual, is much preferable.