The Li-Ion battery should last 6-8 years (current models).
Minor note for anyone buying a 2013 or earlier Leaf:
The Leaf runs the power train off the main battery, but all accessories off a regular car battery. The 2013 and earlier models did NOT connect them; there is an ongoing power drain (think “sleep mode” like computers) that can deplete the 12v battery if the car is unused/uncharged for 2 weeks; when that’s dead, just getting into the car becomes a problem, as does opening the charge port cover.
The 2014 model is smart enough to monitor the drain, and recharges the 12v battery from the main power train battery if need be.
Any estimates concerning what percentage of original capacity exists at that point?
Elsewhere in this thread someone (maybe even you) estimated the battery cost to be about $10K. This translates to about $1,400 per year. This money, at $4/gal, buys 357 gallons of gas, and that is enough to drive 16,000 miles at 45 mpg in a hybrid or an efficient non-hybrid.
This means that Leaf becomes cheaper to drive (assuming electric power of zero cost) if one drives it daily not less than 43 miles. Since power is not free then we can say 50 miles.
It's an interesting balance. If you drive less than 50 miles every single day you will lose money on Leaf. But if you drive more then you have to charge it twice per day or risk losing power on a freeway. There were many articles that claimed that Leaf's range estimate is overly optimistic.
I believe you have a Leaf, so this is not to convince you of anything. (You already know all the facts firsthand :-) This is just a simple calculation that may be of use to someone who considers. I'm living high in the hills, so none of today's EVs will work for me.