How long can an Apple Watch stay without an iPhone?
There's no artificial limit imposed on how long the Apple Watch can remain away. When the Apple Watch reconnects to the iPhone, it will sync any data which is not on the iPhone as usual.

The Apple Watch has 8 GB of internal storage — whilst that won't all be available for Health data, Health only uses a very small amount of storage (~6 months for me has used 3.6 MB) so this isn't something to worry about.