Basically what Leftie said. The reasons for asking about location and needs are to determine range needs and if it's a hot climate (more capacity loss over time than a cooler one).
1st Leafs came out in Dec 2010, but I don't see a reason for the batteries to suddenly fail at the 10, 11 or 12 year mark (for example). They will lose capacity and maybe a lot. The worst I've seen a pic of a 2011 Leaf with 9 capacity bars out of 12 gone, which means probably 35% of capacity remaining. (See table at http://www.electricvehiclewiki.com/wiki/battery-d1/
Battery Capacity Behavior.) But the '11 and '12 Leafs had a crap chemistry. Nissan has changed the chemistry several times since then and the 40 kWh battery is too new for us to know much about how it will hold up. But, there's the 8 year/100K capacity warranty I referred to. Unfortunately, we don't know what the 8 bars represents as the table went away in later service manuals I don't know if it ever came back. Warranty talks about capacity bars, not % capacity remaining.
(Nissan has previously said "the lithium-ion battery for your 2011 or 2012 Nissan LEAF is now also warranted against capacity loss below nine (9) bars (or approximately below 70 percent" at viewtopic.php?t=13192
and something similar at a few other places but AFAIK, none of the US capacity warranties in the warranty booklets have ever indicated a % remaining, only capacity bars.)
I can't think of a single case of pack failure here. Plenty of older Leafs have had packs replaced for capacity loss. A tiny set of people have had a bad module, which is covered by the battery defects warranty (if not expired). The bad module gets replaced, not the entire pack. viewtopic.php?t=21997
has been swapping in packs or modules from newer wrecked Leafs w/batteries in better condition into older Leafs. I could see this happening if for some reason Nissan no longer produces 40 kWh packs. There surely will be some wrecked 40 kWh Leafs.
If you got lucky and got new pack under warranty due to losing enough bars before expiration, then you can probably go for quite awhile longer. And, having charging on both ends doubles the operating radius of an EV. You can also look for charging using https://www.plugshare.com/
. You'd want to filter by J1772 and CHAdeMO (if the car has the larger DC FC inlet on the left.)