I have a MacBook Air mid-2011 11" as well, with 1.6 GHz Intel Core i5, 4GB 1333 MHz DDR3 Ram.
It's running El Capitan well for me.
I'm hoping I won't need to update my computer for a couple more years, since I intend on purchasing an iPad Pro soon, and if my Mac wasn't performing as well as it is then I would probably not be in the market for an iPad Pro since it will cost about a grand.
I thank the SSD in this machine for it performing as well as it has. I've used a lot of my friends' Macs who have iMacs or MacBook Pros with HDDs, and they're painfully slow! I use a completely maxed out 2013 Mac Pro at work, so I don't necessarily need a fast processor for rendering stuff for my own personal use since I'll just be able to use that machine if I ever need to render anything intensive. I mostly just need my MacBook Air for file management, and the occasional photoshop work. I'm sure it will last a few more years with these tasks.
How long do you all expect your Airs to last before you're in the market for another machine?