Exactly enough to fill out the address, which is always the same length. BTW, IPv4 does basically the same thing. The address 127.1 is equivalent to 127.0.0.1.
In IPv4 you also have 127.257 equal to 127.0.1.1, 123456789 equal to 7.91.205.21, and 010.010.010.010 is a well-know DNS server. This notation is also rejected by most implementations.