Originally (2002) a /48 per site was recommended in RFC3177.

More recently (2011) RFC6177 took a more pragmatic / softened approach, but it does say:

      - it should be easy for an end site to obtain address space to
        number multiple subnets (i.e., a block larger than a single /64)
        and to support reasonable growth projections over long time
        periods (e.g., a decade or more).

I don't really understand why ISPs choose to be so stingy with allocations. An extra 8 bits of address space to allocate /56 instead of /64 costs them effectively nothing and has considerable operational benefits, simplifies CPE configuration etc. Just minds still living in IPv4 land I guess.

It's not an RFC, but RIPE690 is pretty clear on the matter:

https://www.ripe.net/publications/docs/ripe-690#4-2-3--prefi...