It depends what your starting point is. An irony of Hibernate is one reason it exists is to try to allow developers to not have to think about the database,
but the more you understand about relational databases the better you understand why Hibernate does what it does. If you are comfortable writing database backed applications, OK with SQL, understand the main concepts of relational databases and OK with basic
Java you should get up and running fairly quickly. Advanced use of Hibernate is fairly involved, and if you are looking to have this as a salable skill in interviews this is the bit I'd be aiming for. The ins and outs of Second level cachinq and the query cache for example, or the choices you can make to improve performance via association mapping decisions and fetch strategies in HQL or Criteria searching. Some knowledge of the various session management and mapping
patterns is a good thing to know, as is knowledge about interceptors.
How long this will take I can't say. Writing example applications with a decent book beside you (and Hibernate in Action is pretty good) is the best way to go. Also, ask questions here. There are a lot of people in this forum with quite a bit of Hibernate experience.