So while IPv4 was basically developed under the idea that every single device on the internet will get its own IP address. When subnetting, you're thinking about how many addresses you have and how many you have to allocate. On the other hand, IPv6 is only worried about how many /64 subnets you have available. Because of the way local networks work, you don't need to think about how many IP addresses will actually be used on any given subnet. Each subnet will have more addresses than they will ever use, so the IPv6 paradigm just doesn't worry about them.
While the basic logic of subnetting is still present under IPv6, there's been a series of conventions that were adopted about IPv6 standard network sizes.
/64 is the basic size of a subnet. Subnets are usually not expected to be smaller than /64, and none ever really need to be larger, since /64 is more addresses than any network would ever have to use.
/56 is a block of 256 basic subnets. Many ISPs delegate blocks of /48 on a consumer level, but have sometimes started to allocate /56 size subnets.
/48 The basic recommended size of blocks that most ISPs should be allocating to consumers.
/32 This is the default block size that ISPs will get when they request more addresses from a regional registry.
Am I wasting too many subnets? Aren't we just going to run out again?
Nobody knows. Who can tell the future?
But consider this. In IPv6 the number of available subnets is the square of the number of available individual addresses in IPv4. That's really quite a lot. No, I mean really quite a lot!
But still: we are automatically handing out a /32 to any ISP who requests one, we are handing out a /48 to every single ISP customer. Perhaps we're exaggerating and we will squander IPv6 after all. But there is a provision for this: Only one eighth of the IPv6 space has been made available for use so far:
2000::/3. The idea is that if we make a horrible mess of the first eighth and we have to drastically revise the liberal allocation policies, we get to try 7 more times before we're in trouble.
And finally: IPv6 doesn't have to last forever. Perhaps it will have a longer lifetime than IPv4 (an impressive lifetime already and it's not over) but like every technology it will someday stop mattering. We only need to make it until then.