What is a typical precision of gettimeofday()?

Over in the thread discussing the addition of UUIDv7 support [0], there
is some uncertainty about what timestamp precision one can expect from
gettimeofday().

UUIDv7 uses milliseconds since Unix epoch, but can optionally use up to
12 additional bits of timestamp precision (see [1]), but it can also
just use a counter instead of the extra precision. The current patch
uses the counter method "because of portability concerns" (source code
comment).

I feel that we don't actually have any information about this
portability concern. Does anyone know what precision we can expect from
gettimeofday()? Can we expect the full microsecond precision usually?

[0]:
https://www.postgresql.org/message-id/flat/CAAhFRxitJv=yoGnXUgeLB_O+M7J2BJAmb5jqAT9gZ3bij3uLDA(at)mail(dot)gmail(dot)com
[1]:
https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis#section-6.2-5.6.1