Still have my 2009 MBP as a backup machine for my late-2013 rMBP. Thought I was going to have to sell or recycle it, but it got some new life with a new battery and the umpteenth SATA cable (had a LOT of those replaced over the years).
I normally never keep things around as backup. After using the faster machine, what is the chance one will want to go back to the old one even for a few days. Normally I sell any tech stuff as soon as I have a replacement. MBP have excellent resale value.
So I guess typically I keep it until the next refresh.
I usually keep mine in excess of 2 maybe 3 years though with the 2012 rMBP I see no reason to upgrade this in the near future. Spending what now amounts to almost 2k is getting harder to justify especially when the change in performance is getting smaller between each iteration.
The retina screen was a biggie but I'll be happy to get 5+ years out of the old girl. I'm already 2 years into it and its going strong.