I've been planning to replace my 2010 MacBook 13" for a few years. Each year it keeps doing everything I need it too, and the new systems don't seem like enough of a jump. I upgraded to 16gb of RAM and a 750gb hybrid drive. It hates doing anything computationally intensive, my IDEs take another few milliseconds to respond, but if I am patient it does everything I need.
I have no idea how long it will last at this point, I think it could be good enough for a few more years. If I put in an SSD it would give it another life.
Same here. My 2011 15" MBP is actually my fastest Mac by many regards. 2.4GHz quad-core Intel Core i7 + pretty nice GPU. I think I put it at 16GB ram, and installed an SSD. I still use it for all of my music stuff, and it doesn't feel slower at all than my 2014 MBP 13" Retina. For what I do with it, I don't see needing to change it at all.
I have no idea how long it will last at this point, I think it could be good enough for a few more years. If I put in an SSD it would give it another life.