I bought my first mac in 2009. A 2 year old 2GB of RAM 15" MBP. Sold it and bought a new 2009 15"MBP. Then bought a new 13" MBA in 2010. Sold the 15" MBP and bought a used 15"MBP in 2011 cuz I wanted quad-core. All good until radeon gate struck. Repaired and sold. Bought new 15" rMBP in late 2013. Amazing machine.
So, I still have a 4 year old MBA but a less than 1 year old MBP. So, 2-3 year vote.
So, I still have a 4 year old MBA but a less than 1 year old MBP. So, 2-3 year vote.