A Rosetta Data API implementation is considered "complete" when anyone can run the rosetta-cli check against your Data API implementation without error (to the current block height). If running the command doesn't halt with an error (correctness checks are documented here), it is considered to be "complete".
Developers assume your implementation passes the checks previously mentioned and will likely not be able to integrate with your implementation if it errors.
After you initially validate the correctness of your implementation, the specification will update, your implementation will change, and more correctness checks could be added to the rosetta-cli.
If the rosetta-specifications are updated,
there is an expectation that your Rosetta Data API implementation should be updated within
some reasonable time to work with the updates. As mentioned in versioning,
1.x release is considered the first stable release and your implementation will not
break from any updates but new features could be added.
If you implement changes to core node software that break your Rosetta API implementation, there is an expectation that you fix your Rosetta API implementation as soon as possible. Many developers will use your Rosetta Data API implementation exclusively and poor update support will cause users of the blockchain you are working on to encounter downtime.
The rosetta-cli may be updated with additional correctness checks based on new behavior restrictions proposed by the community. There is an expectation that you fix your Rosetta API implementation as soon as possible to address any failing checks.